Sounds like a blocked tank vent
When the vent is blocked, air in the tank displaced by incoming waste has nowher to go, so pumping the toilet pressurizes the tank, pushing waste back toward the toilet. Ignored long enough, it can pressurize the tank enough to burst it.All of the above assumes that what's filling your bowl IS waste water backflow. If it's clean water, it's coming in via the head intake. If it's happening when the toilet is in the dry mode, the wet/dry valve has failed. If there's no vented loop in the head intake, it's allowing water to rise in bowl to the boat's waterline. To fix that, you need to install a vented loop in the intake (it goes between the pump and the bowl, btw), and either replace the wet/dry cam assembly, or better yet, the whole toilet. You might find the link below useful in understanding how your sanitation system works and how to maintain it.