my "well over the top" build
http://www.modelboatmayhem.co.uk/forum/index.php?topic=21584.0 my advice, for some extra power at a reasonable budget, Turnigy T600 880kv brushless motor, direct drive, run on 2 x 2 cell lipos, so 4 cells in total, with a big mah, 5000. X52.5 prop, and use M5 shaft, turnigy 60amp reversing brushless esc should be ok, get higher amp if money allows, then you can upgrade to 6 cells, and watch it fly.
motor £25.00
esc £25.00
batteries £15.00 each
prop £1.50
shaft £5.00 depending on length
decent bearings, get SHG to make the shaft, and ask for a glacier bearing at water end, and ballrace at motor end, and an oiler tube, not grease, it will spin too fast
Find a decent coupling, not one of those red things, it will fall apart
I have used this set up in a heavy 36" Aerokits Fireboat, circa 19mph, 15 mins run time, or longer if you back off the gas!
Brushed set up, Graupner 700 BB Turbo, 12v but run on 19.2v nimhs. Electronise esc, the biggest amp one, X 50 prop, or a Graupner 8.4v bb turbo (bb's have better bearings and higher voltage ranges)
Motor £ google it, not sure, circa £20.00 if you shop around, dont buy second hand, and run it in on 3v for at least an hour, regularly oiling the bearings.
Batteries use component shop, again high mah, 4300 or higher
esc £35.00 (I think!)
Again, a tried set up, circa 16mph, 20 mins runtime, unfortunately speed and run time are not bed fellows!
here is the fireboat on the brushed set up, (still have not sorted the spray rails so it will handle better!
)
http://youtu.be/5F4fCe_s-rc 
