Splashed the new to me boat this weekend. Started both motors (502 Gen5's TRS Drives) after launching and saw that the port engine temp was rising fast at 210 shut it down and found a bad impeller. Change it out and both motor temps running at 180. Cruise the lake for an 1.5 hours everything looks good and the boat run fine. Stop at the beach for 2 hours go to start motors, port hard to start, finally starts and BIG knocking sound, shut it down. No water in the oil, going to pull plugs today. I'm guessing a bent rod.

ugh, good luck. The impellor pieces may have got clogged in the oil cooler -

Good luck...but doesn't sound good

I'd guess you filled a cylinder with water (riser/gasket?) and bent a rod when it started. Not tough or expensive to fix normally.

Pulled the plugs today, no water in any of the cyclinders! Spinning it with the plugs out no water, but I can hear a fain "thud sound". with a bent rod, would I still get good compression? I was going to do a compression check on the cyclinders to rule out a bent valve. I can't get to the valve covers with the exhaust in place.

not that it matters but you have gen 5's with TRS, sounds like replacement motors already, are the hours low?

They were replaced, and they have less than 100 hours on them (what I was told).

The temp issue and the impellor issue sux, if you bought from a dealer there is always going to be a question if you ran it with out h20 and burnt the impellor out yourself. Hope its an easy fix

Scenario..
Got hot, combustion temperature got WAY hot, detonated, shoved the oil out between the hot rod and the crank journal (my guess is 3-5, as these are the hottest cylinders) then grabbed the bearing, and damage done, the bearings started shedding trash and oil flow was reduced "down stream". it's surprising that you ran for 1.5 hours before things really went south but that's the way things go sometimes..
The 502's use real nice forged pistons so there may be no evidence of detonation on the pistons, so the bores may be just fine. You'll most likely need to go for a hot tanking, (means honing, and new cam bearings) to get everything squeakey clean, then turn the crank and polish, new rod or two, set of rings, bearings, oilpump and lifters. With luck you'll get away with a hand lapping of the valves unless the valves are tuliped..
This is a worse case scenario... but i've seen this movie before.
Hope yours has a better ending than mine did..
G'luck