Little Charlie - I wouldn't bother going down the IC route again, there aren't many places where you are allowed to use them now.
I bought one of the big Huntsman 31 (though all timber) a little while ago and it is fitted with a brushed Torpedo 850 motor which with a 7.2v lead acid battery (very heavy) has mediocre performance.
I plan on changing to a LiPo battery and a brushless motor and would be looking at something like an Overlander Thumper 5045/10 720 kV. This has plenty of torque to get a heavy boat moving and as you can use 3 to 8s batteries
it should give you all the performance that you want.