http://www.hobbyking.co.uk/hobbyking/store/__67040__HobbyKing_50A_Boat_ESC_4A_UBEC.htmlspecifically made for a boat with enclosed hull in mind. Most cheaper boat esc don't have any or efficient cooling, so they have water cooling such as this. Ok, from international warehouse so check the postage
The 30a version is available from UK warehouse, so your amp draw will be determined taking into account the props and voltage. I would go the 50a route, nothing wrong with plenty of amperage headroom, so find out what else you need and make use of the postage charge.
I have used a number of these, easy to programme using the transmitter, once you get used to the tones
By the way, its one esc per motor, incase you didn't know, and they are reversing, with a nice touch that you can limit the speed in reverse, so you don't accidentally slam full throttle reverse by accident!