I posted this on the Fountain forum also:
I took my boat out yesterday (Fountain 47 with triple 502's) for a "shake down cruise" after 4 months of heated winter storage. The motors started fine, sounded fine, and the guages all read fine but all three overheated after about 15 minutes. I shut them off and let them cool but when I started them again there was no water pressure and they overheated again. I can't believe all three pumps went out simultaneously but I'm not sure what else would have caused it. Gross inspection looked ok although the manifolds were a little hot to touch (not terrible). Intakes also looked clean. Any suggestions?

impellers might have got hard and crusty, then just fell apart, seems hard to believe 3 would do that, but you never know

like I said in fountain forum...

Did you have water pressure initially?

Did you run the engines on the hose first, if so you had at least enough water in the system. Is there a common hose connection for all 3 engines? Check to see if manybe there is a leak allowing the pumps not to pull water.

Are there inlet sea cocks that may be closed?

there are but they were open.

was there water coming out the exhaust?

Hook up a water hose to the line coming out of the water pump and force water backward. Maybe a family of mice thought it was a condo.