Check the tank vent for a blockage...<sigh>...
One MORE time...If the vent is blocked, the tank will pressurized, resulting in backpressure that results in backflow into the bowl.The two most likely places for a blockage to occur are the vent thru-hull and vent fitting and end of the hose at the tank.Don't use the toilet or attempt to pump out again till you're sure the vent is clear.You CAN prevent it from happening: backflush the vent--blast out the thru-hull every time you wash the boat and then send water through back into the tank...do the same thing every time you pump out.Franklin, you've been here long enough that I should have to tell you this again...'cuz I can't even begin to count the number of times I've posted it.