As an 35-year engineer that has designed installed many industrial hot water systems, I can't help but feel you are making excuses for half-fixes on nice boats. Install an expansion tank. It will be safer, stress on the piping will be reduced (you are going over pressure--that is what is causing the valve to open!), equipment will last longer, and the pump will cycle less. It will be better for the water heater.
Just stop dodging the the problem with half solutions and fix it properly. My PDQ has an expansion tank and it works very well. Yes, you can get away without one, but it is not good for it.