This spell is awesome!

Creating a body of rough water on a battlefield changes so many things about combat: movement, vision, weapon attacks, fire, and issues with breathing that affect spellcasting.  Players and DMs will have to keep a cheat sheet to keep track of it all.

Movement

First, this spell is a trap for anyone who lacks a swimming speed (or a flying speed to get above the water), especially if their Strength isn't great.

Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re [...] swimming[.] You ignore this extra cost if you have [...] a swimming speed and use it to swim. At the DM’s option, [...] gaining any distance in rough water might require a successful Strength (Athletics) check. 

So, after the spell requires a Strength saving throw to avoid being pulled 10 feet toward the center, being in difficult terrain water means moving 1 foot costs 3 feet of movement.  And since a "swirling mass" of water sounds a lot like "rough water," the DM may require an Athletics check to move at all, after which a creature with a walking speed of 30 feet could move all of 10 feet with its movement and another 10 feet with its Dash.

It gets better: if a creature lands in the water, or tries jumping up to get its head above water, it may have an even harder time escaping.

When you land in difficult terrain, you must succeed on a DC 10 Dexterity (Acrobatics) check to land on your feet. Otherwise, you land prone.

If a creature does fall prone, it moves at half speed until it uses half of its movement to stand up again.  Its attack rolls are at disadvantage.  On the plus side, attacks against the creature from more than 5 feet away are at disadvantage.  (If a character risks getting within 5 feet, that character's attack rolls against the prone creature have advantage.)

Vision

In a combat between two creatures, at least one of whom is caught in swirling water covering the 5-foot cube they inhabit, there's a good chance the creatures are at least sometimes lightly obscured from each other.  That disadvantage on Perception checks means it's relatively easy for them to hide from each other, or just attack unseen.  The rules on unseen attackers and targets say that attackers have advantage on attack rolls against creatures who can't see them; on the other hand, being unable to see your target imposes disadvantage on your attack.

Weapon Attacks

As if vision didn't create enough issues, it's a lot harder to hit things through the water with most weapons.

When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.

A ranged weapon attack automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).

Fire, and Vulnerabilities to Water

The previous link also mentions, "Creatures and objects that are fully immersed in water have resistance to fire damage."  Naturally, all this swirling water would also put out any existing fires (and prevent things like Fireball from lighting objects on fire).  Between that and the usual cloudiness of swirling water, the level of light in the area could be dimmed, with consequences for combat.

Naturally, being immersed in water is the worst news for a fire elemental or fire elemental myrmidon, who take damage from just being exposed to water.  A 5-foot cube of water contains 935 gallons, more than enough for instant death. 

There's a strong argument that this water counts as "running water" for the purposes of ruining a vampire: dealing 20 acid damage every time it ends its turn in the water, and preventing the vampire from using its shapechanging, misty escape, and regeneration abilities.

Breathing and Spellcasting

Most water spells aren't persistent enough to create an issue with breathing, but Maelstrom is.  Creatures who can't hold their breath can get into big trouble quickly by suffocating.

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

Almost any combat is over before 42 seconds (7 rounds) have passed, but if a creature in the swirling water doesn't hold its breath and has a CON modifier of +1 or lower, like most humanoid mages, it could be knocked out in two turns.  And being unconscious in a Maelstrom is extremely punishing, because unconscious creatures automatically fail Strength saving throws (so at the start of their turn they take the 6d6 bludgeoning damage from this spell and automatically suffer a failed death saving throw, and get pulled toward the center of the water so they're harder to extract) and can't be stabilized or healed there.

What could cause a creature to stop holding its breath?

• Again, anything that knocks the creature unconscious.  Sleep or badly failing a saving throw on a dose of drow poison or a sprite's arrow may only knock the creature unconscious for a moment before the jostling and damage of Maelstrom wake it back up, but that's enough time to stop holding one's breath.
• Arguably, anything that paralyzed a creature.
• Something that makes the creature laugh, like Hideous Laughter or cackle fever.
• An item or effect that gets the creature choking/suffocating ahead of time, like dust of sneezing and choking.
• Talking, or casting spells with vocal components, which is the vast majority of spells.  (Here are the 27 exceptions.)

To make matters worse for spellcasters, concentrating on a spell is difficult in the swirling water.  Not only does the damage force concentration saves, but there's every reason to believe that just being tossed about in a whirlpool would force another concentration save:

The DM might also decide that certain environmental phenomena, such as a wave crashing over you while you're on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.

Summary

This spell could be terribly hard to escape, and especially dangerous against spellcasters and ranged attackers.  A DM could use this to seriously threaten players with low Strength and Constitution.  A mid-level druid with a couple of sprite allies could be an unexpected deadly threat.

With the right ally, a downright cruel combination of spells would be Maelstrom and Sickening Radiance. 

• If the DM agrees you need an Athletics check to move at all in the rough water, taking just that first level of exhaustion (imposing disadvantage on ability checks) would make it hard to subsequently escape. 
  • Likewise, anyone who jumped up for air or landed in the rough water would have disadvantage on the Acrobatics check to avoid falling prone, which would make it harder to escape on the next turn. 
  • If someone inside the area of effect risked casting Dispel Magic on either effect, but didn't cast it at a high enough level to automatically cancel the spell, they'd have disadvantage on the spellcasting ability check.
• If the creature doesn't escape quickly, its fate may be sealed by that second point of exhaustion, which halves its movement speed.  That's a terrible handicap when you already have to spend 3 feet of movement to move one foot.
• A third level of exhaustion means disadvantage on the saving throws, which is crushing: from then on, the creatures would be more likely to be pulled ten feet toward the center, more likely to take another point of exhaustion, more likely to take a pile of magical bludgeoning and radiant damage, and more likely to fail any death saving throws.  At that point, the chances of escaping before death are near-hopeless, because that's a ton of damage, and the next three points of exhaustion are: hit point maximum halved, speed reduced to zero, and death. 
  • The third level of exhaustion also means disadvantage on attack rolls, in case it matters. 
• Anyone who fell unconscious in the meantime would quickly stack up failed death saving throws and risk suffocating before they could be stabilized or healed.
• And the whole time, being bathed in that dim green light would make it harder to hide (and impossible to turn invisible), so the creatures in the spell area would be easier to hit as they tried to escape.
• Being caught in the water would make it difficult to interrupt the concentration of the offending casters, who could cast both of these spells from 120 feet away—outside the normal range of any weapon that can be fired or thrown from the water, and beyond the range of most spells (including Counterspell by a fair margin).

Good news! Maelstrom is a 5th-level spell, so you unlock it at 9th level.

I want to put a constant version with a portal to the elemental plane of water at the center. This could create a good way to get characters to the plane of water, if they explore aquatic ruins,cor go through a cult of elemental evil.

Quote from Durandals_Fate>>

So, after the spell requires a Strength saving throw to avoid being pulled 10 feet toward the center, being in difficult terrain water means moving 1 foot costs 3 feet of movement.  And since a "swirling mass" of water sounds a lot like "rough water," the DM may require an Athletics check to move at all, after which a creature with a walking speed of 30 feet could move all of 10 feet with its movement and another 10 feet with its Dash.

Just want to clarify for anyone reading this that there's no reason why this spell by itself would cause movement to cost any more than 2 feet per 1 foot traveled. The spell makes the area difficult terrain which means it takes 2 ft of movement per 1 ft traveled. The fact that it mentions water in the description doesn't mean that it costs an extra foot of movement on top of the difficult terrain created by the spell. Presumably the area of the spell is difficult terrain because of the swirling water but the spell doesn't specifically link the difficult terrain to the watery conditions. That doesn't mean we should apply extra movement restrictions based on movement in water since the spell does only what it says it does. Instead, it means that even creatures with swimming speeds will treat the area as difficult terrain so DMs don't have to worry about differentiating walking v swimming speeds when applying the spell. 

The spell can still be used to reliably lock down movement though, particularly in combination with spells like plant growth, transmute rock to mud, or any spell that reduces movement speeds. Plant growth + maelstrom + failed strength saves = creatures are stuck in the aoe (so long as their movement is 30 or below and they don't have a fly speed). Add in any spell/ability that reduces creature speed by at least 10ft and creatures will be immobilized within the maelstrom even if they don't fail the save.

I love this spell so much! I just wish it had greater distribution across the other spellcasting classes.

Quote from NH23 >>

Quote from Durandals_Fate>>

 

So, after the spell requires a Strength saving throw to avoid being pulled 10 feet toward the center, being in difficult terrain water means moving 1 foot costs 3 feet of movement.  And since a "swirling mass" of water sounds a lot like "rough water," the DM may require an Athletics check to move at all, after which a creature with a walking speed of 30 feet could move all of 10 feet with its movement and another 10 feet with its Dash.

Just want to clarify for anyone reading this that there's no reason why this spell by itself would cause movement to cost any more than 2 feet per 1 foot traveled. The spell makes the area difficult terrain which means it takes 2 ft of movement per 1 ft traveled. The fact that it mentions water in the description doesn't mean that it costs an extra foot of movement on top of the difficult terrain created by the spell. Presumably the area of the spell is difficult terrain because of the swirling water but the spell doesn't specifically link the difficult terrain to the watery conditions. That doesn't mean we should apply extra movement restrictions based on movement in water since the spell does only what it says it does. Instead, it means that even creatures with swimming speeds will treat the area as difficult terrain so DMs don't have to worry about differentiating walking v swimming speeds when applying the spell. 

 
There is a reason: "Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re [...] swimming[.]"  If you're a Medium or smaller creature in 5-foot-deep water that's swirling so hard it's bludgeoning you for 6d6 damage, you're probably swimming.  And the spell says this water-filled area is difficult terrain.

I wanna make this then shape shift into a water animal

wait this will affect the caster nvm

Just cast this spell in underwater campaign.   Underwater casting doesn't seem as powerful.   30' radius is nice, but is it correct enemies can simply swim 5'+ up and escape the whirling maelstrom?

Technically, yes, I guess it could be interpreted that way, but that’s not very fair to you, and is a shitty call to make IMO. You should be getting your money’s worth on a 5th level spell, and it seems kinda lame to rule that it has no effect on the 3rd dimension underwater and everything can escape it within the first turn of the spell’s effect.