Pirate ship

Location Sea
HP 111
Damage 2–7
Notes Range 3 attack
You may expect the Pirate ship's cannons to be very powerful, but you'd be mistaken. All the Pirate ship really has going for it is a lot of HP. Too bad you can't Board them like in Ultima II.
Compare Pirate ship to other Creatures
Dragon turtle Hood

Related

Frigate

HP 160
Appears on Sea
Notes Can be Boarded, travels on water
Frigates are a godsend when they appear on overworld maps, presuming that you have a Blue Tassle. You can walk right onto the Frigate and Board it to take it for your own. Interestingly, the actual enemy Frigate remains there, so soon you'll be using a Frigate against itself... weird.

The fact that you can Board enemy Frigates is probably a bug, though likely the sort of bug that was noticed and left in the game intentionally. You can complete the game without taking enemy Frigates, but it's not nearly as convenient to do so.

Pirate

Experience 5
Appears Sea
Notes Appear as thieves in combat
Pirates are a nice enemy to have, because once they're defeated, you can take their ship! Pirate ships do tend to sit back and fire their cannons at you, though, so you'll need to draw them in before attacking. In actual combat the Pirates appear as Thieves, and you'll have to board their ship. This is a major advantage for you since there are two chokepoints that you can plug with a melee character while you have ranged Attacks firing from behind.