With electric motors, and the rest of the components, think of heat as inefficiency and something to be reduced, not just an unavoidable byproduct. This is not a hard and fast rule, just a guideline. Most parts have operating parameters and you really need to know and respect them. But for powering a sailboat with reasonably priced components (motor, controller, etc.) you should be able to design a system where excess heat is not a problem. I'd bet dollars to doughnuts that if you have enough heat that you need water cooling, you can find more appropriate parts that only need air cooling.
A lot of the DC motors I'm used to will run hot because we're running 75 to 300 amps continuously with spikes in the 500-700 range. The motor gets hot enough that you almost can't hold you hand on it. In this case, the heat is a product of pushing the motor hard. And it's still within specs (insulation rated for 180C.) But if you are only motoring in and out of port, and your motor is getting hot, you may want to consider a more appropriate motor. Remember also, you can use a larger motor and not pay any price in efficiency or energy used.
Look for a motor that is designed to run at the speed your prop should turn. You can also work backwards from the ICE motor specs and find an electric that will fit the profile. And if you need to "put your pedal to the metal", or run hard to get out of rough seas, an over sized motor will give you a little more margin.
Dave Cover, still looking for my project to convert
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