It's not surprising that a large number of menacing water types will be pirate-y.
For the purpose of consensus: Gyarados, Sharpedo, Tentacruel(?), Blastoise, Octillery can all be part of the sea monster/arsenal aspect. Wailord and Avalugg can be your ships!
Things like Kingdra, Gardevoir and Milotic can also add a touch of romance to the aspect, because many a pirate/privateer was known to go in for that sort of thing *clears throat primly*
Obvious fire types include Infernape, Houndoom and maybe even Chandelure (some of those pirate ships had fancy lights). Ambipom is also a simian candidate.
A large number of fighting, flying and dark types are also plausible: Chatot being very obvious, but also Staraptor, Honchkrow, Pelipper, Primeape, Scrafty, Malamar, etc.
Not all pirates were barbaric or crude. Serperior, Mienshao, Empoleon and Gallade can be the refined, somewhat Frenchified pirates, if you want them to be. (and Serp gets you a free Boa Hancock reference.)
A collection of things, like Luxray, Krookodile, Bisharp, Barbaracle/Kabutops, Weavile and Haracross can also go okay enough with the theme, but more importantly they add a good type diversity.
All the best, and may the wind rise under your sails!