Hi
I have a Aerokits Sea Commander with a 35mm brushless motor
3639-1100kv Turnigy 800 watts, 3S Lipo battery 5800mah 25C Turnigy, 60 Amp car ESC and using a 2 blade 40mm prop. Good turn of speed without going silly.

Hi Adijohn
Welcome to the site.
You can search the site for Sea Commander and will find how others have progressed with their builds.
Regarding brushless I would expect a 35mm outrunner to be a suitable size with a prop of the same diameter and direct drive. The entry level power supply would be a NiMh battery of about 5000mAhr at 9.6 to 12v. if you really want the speed and endurance then LiPo is the best but does cost more and needs correct handling both when sailing and especially when charging.
Brushless motors have kv ratings which indicate the free load speed per volt, so for an 800Kv motor at 10v = 8000revs (it will be less under load). They also have a wattage limit which is the product of the voltage times the amps, so for 10v at 20 amps this is 200 watts. if you exceed this the motor will overheat and fail. The ESC also has a max amp rating and best practice suggests this is twice the expected running current.
You can buy a wattmeter to measure current and watts.
There are many suppliers but have a look at http://stores.ebay.co.uk/Component-Shop?_trksid=p2047675.l2563.
Hope this helps and if you like my response please tick the like box to help others find the post.
Good luck and please post details of your build.
Dave