I had a HP500 in my 29 Fountain and when I sold it, it had 250 or so hours on it. The guy I sold it to ran it for some more hours and then tore it down to change it over to a blower motor. He told me the engine looked like new inside, everything was in perfect shape, springs, valves, bearing, cam and crank shaft, water cooling passages showed no signs of salt water and this had to be clase to 300hrs and all in salt water.
During my time of ownership I changed over to Mobil 1 synth oil at 20hrs and changed the oil every 30-40hrs which for me was around 4 times a year. I also flushed with Salt-A-Way every time or at least every other time.I changed plugs and distruiber cap and rotor every season.
Enjoy your HP500's, they are great engines!