The nominal voltage of the motor is 9.6V (8 cell operation) - if you are running on 12V the currents seen will be a little higher - the stall current will go up to 150A - the other currents will go up similarly.
With modern ESCs, the heat output wont be that much on a motor like this - for example an ACTion Condor 20 on full load will dissipate about 1 watt in each of the output transistors. This will give a temperature rise on full load of about 30C - well within the scope of most boat interiors. Water cooling will not really add anything.
You can gain a bit of extra cooling by mouinting the ESC low in the hull - if the esc has a heatsink case, just mounting it on the hull will give a few degrees of extra cooling just because of the cooling effect of the water over the hull - but fuse very carefully - if the ESC goes into meltdown it could burn through the bottom of the hull. Not possible with the ACTion ESCs but then it is probably unnecessary.
Fusing is important - the fuse is there to protect the ESC from a stalled motor. It shouldn't fuse in normal full power running, but should only just stay running - any increase in current should cause the fuse to go. Use the closest value above the normal operating current you can. Remember the rating on the fuse is not the current it blows at but the maximum current that it will allow to flow without blowing. How long it takes to fuse after that depends on the overcurrent - for example blade types fuses will carry a 10% overcurrent for 100 hours typically and a 35% overcurrent anything up to 10minutes. The fusing time will only fall below 1 second for an overcurrent of 250% (i.e. 70Amps). If oyu use a single fuse put it on the input to the ESC - if you have space, put fuses on the input and the output of the ESC. Have the one on the input of the ESC the next size up at least to the one on the output. The output fuse protects the ESC against the motor stalling out, the input fuse protects against the ESC failing. This is not as important with an ESC that uses relays (e.g. ACTion Condor 20) as failure of the output transistor will drive either full speed or no speed. With a bridged ESC (e.g. MTronics, ACTion P93) if there is a transistor failure there is the danger of shorting the supply causing meltdown or even fire. ALWAYS ALWAYS fuse the input to the ESC
Wom