martin18,
Basically, the size of the ESC is determined by the maximum current draw of the system. Find, or figure out how much current one motor draws, double it and you have the current requirements for two motors (if they're connected in parallel). A 'stall test' will tell you what each motor's current draw is. Add a little "fudge factor" to that current total and that's the 'size' ESC to get. It's always better to have a little 'too big' of an ESC, than one not big enough.
For boats, a 'water proof' ESC is nice. Not always needed if you position/protect them so they won't 'normally' get wet, and the boat doesn't ship a lot of water for whatever reason. Two of the ESCs I know are available in the UK are 'Mtronics' and 'Electronize' (spelling is optional if I didn't get those right). There are certainly others, I just don't know what they are since I don't normally 'shop' in the UK (4500 miles is a little far to drive).
Good luck.
- 'Doc