using what nick has suggested you can get both the motor and speed controller for £39 inc pnp from here:
http://www.hobbyking.com/hobbyking/store/__18232__Turnigy_D3542_6_1000KV_Brushless_Outrunner_Motor.html
http://www.hobbyking.com/hobbyking/store/__7360__Turnigy_Marine_60A_Brushless_Boat_ESC.html
ok the the 36-35 and the 1000kv is the type of motor to go for. 3635 is the diameter and lenght in millimeters of the motor althought im not sure if nick has written it like that to give a round about size of the diameter.
The kv is revolutions per volt. usually the lower the kv the higher torque the motor but the slower it runs.
1 lithium polymer cell or lipo is 3.7v. If you were to buy a pack with 2 cells in it it would be called a 2s battery, if it had 3 cells in it it would a 3s battery and so on. dont know why they call the cells S.
a 3s lipo battery would there for be 3 cells of 3.7v totalling to 11.1v
if you then take the 11.1v and multiply it by the KV of the motor in this case 1000KV then it would give you 11,100 rpm at the motor spindle.
lithium polymer batteries have pro and cons like all batteries, for startes they need a balance charger. balance charger charge the cells individually this stop them from over charging as a overcharged lipo explodes. like this lol:
http://www.youtube.com/watch?v=6vYn2lbBh0Q&feature=relatedSo a balance charger is vital!!
they also cannot be discharged below 3v as it damaged them, so an LVC or low voltage cut off stops this. It shuts down the forward motion of the boat for a few seconds then allows for a slow recovery back to the bank.
if you like you can save all this hassle of lipo batterues and get a 12v nimh battery pack. you can charge these will a normal charger and you can run them flat, but they dont make your boat as fast as a lipo can. Nimh is also cheaper, but there heavier. depends on what you want to spend on batteries.