An island? That would allow you to lay in wait, and open fire as enemies passed by?
Some sort of sea animal? A giant whale, turtle, or something similar, non-threathening. Would allow you to move without raising suspicion.
A big fat merchant ship, laying low in water, heavy with plunder, and without any kind of defensive weapons. Other merchantmen would feel safe, and other pirates would be drawn into an ambush.
As fair as I read it, you an change appearences, and doesn't have to stick to one. A chelian wargalleon for when you want to avoid combat, a merchantmen when you want to attract trouble. A sinking ship in need of assistance to lure in those silly do-gooders from Andorra, and an island or bank of heavy fog when you want to hide.
Fog bank sounds ideal for hiding.
Kelp island (think "Sargasso") is a nice idea too.
And as an Iceberg.