My players are going to be taking a sea voyage soon and I'm planning some encounters.

Most of the suggested encounters at open sea include native aquatic creatures like Kuo Toa, Merfolk, Merrow, Sauhagin, giant sharks and the like.

How would such aquatic creatures go about attacking people on a ship? Assume there's a pretty good vertical obstacle to over the hull and onboard. I'm struggling to think of how I could make such an encounter interesting.

Thanks in advance!

If it's a full-sized ship, animals like sharks just won't, unless they're big enough to attack the ship itself. But players who have to leave the ship in small boats for whatever reason are targets.

As for humanoids, if they do it a lot, they'll probably have tools for climbing up the side of the hull quickly. (If the ship is anchored, the anchor itself is a means of entry.)

What is their motivation for attacking the ship? They'll have different strategies if they're engaged in piracy than if they want to destroy all surface-dwellers.

A couple of specific ideas that occur to me are:

• Once on board, grappling air-breathers and jumping over the side.
• Riding alongside on giant flying fish, hurling javelins at the crew every time they fly into the air.

Sauhagin like to climb aboard the ship and attack people on the deck.  Most aquatic races will do things like using nets and ropes to foul the rudder and slow the ship.  They could also attack the hull itself to try and sink the ship but this can be a rather annoying tactic because it's extremely difficult for parties to counter, particularly at low level.

Native sea creatures typically don’t need to breath air. Why would they jump onto the deck of the ship and fight enemies in their element? Instead, they can hang out under the ship, knocking a hole or three in the hull while they have total cover from anyone on board. Then, when the ship sinks, and all the land-based folks are flailing around and drowning under the weight of their armor, that’s when they attack. Or just swim off a little way, ignore the people completely, and wait for the ship to hit bottom before looting it at their leisure. 

Quote from Xalthu >>

Native sea creatures typically don’t need to breath air. Why would they jump onto the deck of the ship and fight enemies in their element?

 In-universe because they're bloodthirsty and/or seeking honor and glory in combat, which is typically gained by being bold and aggressive.

Out-of-universe because foes that simply sit back and attack the party from a position where most of them can't retaliate make for a very un-fun encounter.

Quote from 6thLyranGuard >>

Quote from Xalthu >>

Native sea creatures typically don’t need to breath air. Why would they jump onto the deck of the ship and fight enemies in their element?

 In-universe because they're bloodthirsty and/or seeking honor and glory in combat, which is typically gained by being bold and aggressive.

Out-of-universe because foes that simply sit back and attack the party from a position where most of them can't retaliate make for a very un-fun encounter.

They can retaliate, they just have to do it on the enemy’s terms. Seems to me like characters jumping in to swim with the sharks and fight the saughin would be a very fun, and memorable, encounter. 

Depends on the party.  Underwater combat can range from "super easy, barely an inconvenience" to "quick and painful TPK" depending on party level, classes, and what spells and weapons they've got.

I would imagine it looks a lot like the scene in the first Pirates movie when the undead crew of the Pearl attack the British ship waiting outside the skull island. 

These are some great ideas! Thanks!

Other things to use potentially - 
seaweed ropes and grappling hooks to clim up the sides quickly
Whales ramming (ala moby dick) maybe with a party jumping off at impact (an oceanic version of the Dune worm riders?)
ship grappled by a giant squid/kraken with the “minions” swarm in up the arms 

what are they after? If you think about it most surface goods aren’t of much use underwater maybe some foods briefly and metals especially bronze and brass and mithril. They can’t be melted underwater but hey can be hammered into new shapes. Then surface created magic items. If they just sink the ships without knowing what is aboard they mostly get junk  so fighting the crew and passengers let’s them see what is available as well. It may be a case of being “ driven off” because they actually quite as the spoils were not worth the efforts.

I would second that the optimal tactic would be for water breathing adversaries to cut a hole in the hull and watch the ship sink. I'd also second that overcoming this narratological problem is the biggest difficulty in planning combats on the water... What if you include some pirates who have their own ship?

Wooden ships, which is what most people will be using, generally don't sink. You can break them apart, you can render them almost unusable by flooding parts of it, but unless there's a really heavy cargo or masses of metal armour or something, they won't sink from merely having been holed.

I would have it were there are waves of enemies boarding the ship using grappling hook-ish things. They attack the party, hoping to loot them. If you are thinking of a singular enemy, like one of those giant jellyfish from CotN could wrap around the ship and try to pull it down. They could attack the individual tentacles as the encounter.

Quote from Linklite >>

Wooden ships, which is what most people will be using, generally don't sink. You can break them apart, you can render them almost unusable by flooding parts of it, but unless there's a really heavy cargo or masses of metal armour or something, they won't sink from merely having been holed.

Except that they are ballasted with weight such as rocks to lower the center of gravity to stop them from rolling. So when holed, a wooden ship will sink, unless it is of a design like a trireme that does not use ballast.

An encounter: sailing ships require a crew of sufficient size to run both day and night. The crew work in four hour shifts. If there are not enough crew to man the ship during the night then it has to stop while the crew rests. 

have it so that the crew is under strength and needs to anchor for the night. The enemy can be a local underwater tribe or raiding party that attacks while everyone except for a night watch is sleeping

Mounts are a nice way to board quickly. Sea lions can explicitly jump quite high out of the water.

The seafolk I used last were abductors. They primarily tried to net party members and haul them back into the water. The two characters that had good aquatic options really enjoyed the opportunity to rescue the people dragged overboard, and everyone else was engaged on the deck.

Strange things floating in on storms can be fun too. Stuff like will-o-wisps or exhausted flying creatures blown out to see looking for anywhere to rest.

Quote from Linklite >>

Wooden ships, which is what most people will be using, generally don't sink. You can break them apart, you can render them almost unusable by flooding parts of it, but unless there's a really heavy cargo or masses of metal armour or something, they won't sink from merely having been holed.

Plenty of wooden ships have sunk (that's why diving for shipwrecks is a thing) though it's not going to be a fast process. However, even if it doesn't sink all the way, it can easily sink by enough to be no longer actually usable as a ship -- everything below decks is flooded and the ship is unable to move -- at which point the undersea people can come back and their leisure and pick off the remaining crew.

I see no reason aquatic creatures would be silly enough to actually fight the crew of a ship. 

Bring a big saw. Saw off the keel and rudder. Watch boat capsize and sink (throw some grappling hooks to help if need be - have a nice, big sea monster do the hauling). Take anything you want from the wreckage.

It's really on the crew to find ways to defend themselves.

Xalthu said basically the same. 

But assuming you're not looking for actual logical tactics, but a flashy encounter, how about: Cast entangle on seaweed to foul up the rudder, I'd say a Wind Wall placed correctly would keep the ship from using it's sails (or maybe the ship would sail right through, there is momentum to take into account after all), for that matter a wall of water would allow the water guys to swim right up onto the deck of the ship. Summon some water elementals. Use Thunderwave to push defenders off the deck, into shark infested water. 

If it were me I would have them use nets and grappling hooks, but not to foul the rudder or to climb aboard but instead to snare the crew and drag them into the water.

Awesome suggestions. Thanks!

Alternatively, I would strongly consider attacks from flying creatures - harpies, wraiths, ghosts, giant vultures, manticores, rocs, dragons, etc...