"The next thing I'd like is one that runs intermittently like on off on off on off to replicate a real boats exhaust at idle. "
I might suggest adding a riser to the exhaust pie. As the pump pushes H2O up the inboard side of the riser, it finally crests over the top of the riser and spills out the exhaust outlet. The pump meanwhile refills the inboard side of the riser until the H2O again spills over. Very prototypical hopefully. I've never tried it, so let us know if it works.