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.
Compare Frigate to other Creatures
Worlds: Where Frigate may be Encountered – 13 shown
Extraterrestrial planets are external to Earth and only have one time period each
World
The year or time this world exists in, as described by the world's sign
Era
The first coordinate entered when using Hyperspace to travel to this world
Xeno
The second coordinate entered when using Hyperspace to travel to this world
Yako
The third coordinate entered when using Hyperspace to travel to this world
Zabo
Legends Legends - - -
Pangea 9,000,000 B.C. 6 6 6
B.C. 1423 B.C. 6 6 6
A.D. 1990 A.D. 6 6 6
Aftermath 2112 A.D. 6 6 6
Mercury - 5 4 5
Venus - 3 3 4
Mars - 6 2 3
Jupiter - 1 3 4
Saturn - 2 8 5
Uranus - 9 4 6
Pluto - 0 1 4
X - 9 9 9
Minax Guard

Related

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.

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.