Hi mermod,
GO BRUSHLESS! Here in the UK you can buy even a 6 KILOWATT brushless motor for under £100, so you should be able to find an what you need for a good bit less, with brushless you can enjoy zero brush and commutator friction/wear because there are none, higher electrical efficiency, higher watts per gram motor weight and size and more watts per dollar motor cost, true you have to buy a new brushless ESC but a big boat deserves a proper powerplant, good luck and please don't spend a fortune on heavy, underpowered and overpriced conventional motors...unless you really want to! Do let us know how the re-engining goes!
Andy