Looking under Mike's avatar he appears to be based in Canada, not the UK. Hence why I recommended CC as they're a bit closer to home than Mtroniks.
Secondly, Car esc's work very well in boats, and if you want them to run with watercooling, not difficult to modify, although they work perfecly well air cooled, especially if you overrate the current.
Also unlikely you will find a sensored marine ESC, all the ones I have seen are sensorless, which is fine for most uses as boats essentially have a fluid clutch, but if you want very good low speed control, then sensored is better. That means a Car esc, and in fact neither Castle Creations or Mtroniks can help with a sensored controller. The best value can be found at Hobbyking, they do a 60A sensored ESC (which can also run in sensorless mode) for $26. I have one of these, and so far it's proving to be superb, however I would be cautious in recommending it to some, as it comes sans instructions, and you need to update the firmware to get the best out of it. Also I've only been running ti for a short time, so it could yet expire, we'll see.