Yep, water cooling is necessary. My experience is that both motor and ESC gets very hot in a boat, despite being run well within its maximum amp limits. Because air circulation is just not enough.
In a plane, both motor and ESC will have fresh inrushing air at high speed, when set up correctly.
Within a boat hull, you will have slow circulating air, and it wont be fresh cool air.
So my advice is definetly get a watercooled ESC, and I would advise a watercooled motor aswell.
I toasted a 150A Turnigy K-Force in my previous boat, running it at 112A.
The same regulator worked fine at 146A in a ducted fan setup previously. Because cooling was so much better in the plane.