Yes, the problem is that the paddlewheel is either open-collector (or open emitter, can't remember), and the perfect pass module was pulling up(or down) against my speedometers. You can actually see it because the output signal drops substantially. At some point either PP or the speedos can't resolve the signal and (on mine anyways), the perfect pass would cut out for no apparent reason, or the speedos would fluctuate wildly.
I'm a techie, so I problem-solved this by bringing a scope into the boat and measuring the paddlewheel output while underway. Once I figured out what was going on, I built an isolation circuit so that the speedo signal to the PP is buffered from the speedo signal to the speedometers. Not a glitch since, and signal is strong at both ends.