Deep-Sea Stalker

Few rangers are better suited for underwater environments than the deep-sea stalker. Many of this conclave share an aquatic heritage, but some terrestrial members also join upon their attraction for the depths of the sea. Deep-sea stalkers are at home in deep bodies of water, able to hold their breath and swim like the aquatic creatures they track, and can summon the creatures of the deep to aid them in their explorations.

Taken and reworked from Torrents of the Spellhoarder by Elderbrain.com.

Level 3: Home in the Sea

Starting at 3rd level, you have acclimatized to your underwater surroundings, granting you the following benefits:

• You can speak, read, and write Aquan.
• You gain a swimming speed equal to your walking speed. If you already have a swimming speed, it increases by 10 feet.
• You can breathe both air and water.
• You gain 60 ft. Darkvision. If you already have darkvision, it increases by 30 feet. 
• Attacking underwater does not impose disadvantage.

Level 3: Underwater Instincts

Starting at 3rd level, you have honed your senses and hunting methods for aquatic foes. You gain the following benefits:

• When you deal damage to a creature marked by your Hunter’s Mark that has a swimming speed, you deal additional damage equal to your Wisdom modifier (minimum +1).

• You have advantage on Wisdom (Survival) checks to track creatures that have a swimming speed, and advantage on Wisdom (Perception) checks while underwater.

• At 9th level, you can comprehend and verbally communicate with beasts that have a swimming speed. Their knowledge is limited by their intelligence, but they can share information about nearby locations, dangers, and creatures they have perceived in the past day.

Level 7: Abyssal Resilience

The crushing depths no longer daunt you. You gain the following benefits:

• You gain resistance to Cold damage. If you already have resistance to cold damage, you gain resistance to Necrotic damage. 

• You ignore penalties from deep water, frigid temperatures, and strong currents.

• Whenever you roll a saving throw against the Frightened condition while underwater, you make the roll with advantage.

Level 11: Call of the Sea

Starting at 11th level, you can summon a creature of the sea to aid you and your companions while swimming or underwater. You can cast the conjure animals spell without using spell slots or material components. Whenever you start casting the spell, you can modify it so that it doesn’t require Concentration. If you do so, the spell’s duration becomes 1 minute for that casting.

Once you use this feature, you can’t use it again until you finish a short or long rest.

Level 15: Sea Form

Starting at 15th level, your maneuverability underwater has become unrivaled. As a bonus action, you gain the following benefits for 1 minute while underwater and not wearing heavy armor: 

• Your swimming speed increases by 30 feet.
• You can take a bonus action on each of your turns in combat. This action can be used only to take the Dash, Disengage, or Hide action.
• When you take the attack action, you can make three attacks.

Once you have used this ability, you cannot use it again until you take a short or long rest.