Good ideas, everyone. I've thought up of a few more:
Dual Burner
Lv50 War Frigate
Usually come in pairs. If one of them are fired, both will come at you.
Uses: Firebrand as both Broadsides and upper deck guns.
Navy Corruptor
Lv59-61 War Frigate
A Navy Corruptor can do some serious damage to your ship, and has long range ( Like the EITC Tyrant ). However, they are not very fast, and can be easily avoided. Usually carries Royal Chests.
Uses: Firebrand as the Broadsides and Chainshot as the upper deck guns.
Flagship
EITC Pathfinder
Lv79-81 Ship Of The Line