Kraken, giant squid, giant octopus etc. I've had the most fun with these monsters by deploying them as minor encounters all over the ship: for example, this tentacle is tearing down the main mast, kill it fast! This tentacle is snatching sailors off the deck, kill it! This tentacle has started fires, put them out! The squid's beak is eating through the hull, repair it!
Characters can also damage objects with their weapons and spells. Objects are immune to poison and psychic damage, but otherwise they can be affected by physical and magical attacks much like creatures can. The DM determines an object’s Armor Class and hit points, and might decide that certain objects have resistance or immunity to certain kinds of attacks. (It’s hard to cut a rope with a club, for example.) Objects always fail Strength and Dexterity saving throws, and they are immune to effects that require other saves. When an object drops to 0 hit points, it breaks.
Only if the ship actually sinking is an outcome you're happy with. You could instead measure degrees of success by the number of crewmen killed, cargo lost, damage done, etc. The voyage might take longer (and be more vulnerable to the weather or other predators) unless repairs to the sails etc. are made, or the PC's can think of other ways to help. The fight as a whole could have revealed something surprising about what the ship was carrying, or who is on board. If you do decide to wreck the ship, have the survivors wash up somewhere for extra adventures!That sounds awesome!
Yes! That's exactly what I'm thinking. Some huge multi-armed creature wreaking havoc all over the ship. I guess I'll need to assign the ship HP?
Only if the ship actually sinking is an outcome you're happy with. You could instead measure degrees of success by the number of crewmen killed, cargo lost, damage done, etc. The voyage might take longer (and be more vulnerable to the weather or other predators) unless repairs to the sails etc. are made, or the PC's can think of other ways to help. The fight as a whole could have revealed something surprising about what the ship was carrying, or who is on board. If you do decide to wreck the ship, have the survivors wash up somewhere for extra adventures!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.