It’s the water pump at the back of the toilet that makes most of the noise. You can test this by shutting off the raw water seacock and flush the toilet until the water stops. You’ll notice how much quieter it is then. I wrapped the base area around the base of the toilet, especially at the back around the water pump cover, with several layers of insulation and sealed it all up with wide white duct tape. It’s still noisy but tolerable now.
Another option is to use the external pump kit from Raritan and move the pump back into the engine compartment. Or I suppose you could disconnect the power to the toilet raw water pump and just use water from the sink for flushing. Or switch to fresh water flushing from the freshwater pump already on the boat.
Curt