Have a 1998 Rinker with a mercruiser 5 litre 350 that has water in cylinders 5,6,7 & 8 - would that be both the exhaust manifolds or something else?

Sounds like a likely suspect, but have also seen blown head gasket do the same. Was it like this after winter layup, or did the water get in there after running it? If after the layup, then I would suspect rainwater.

Had a bad intake gasket (water in oil) replaced gasket - boat ran fine with muffs - put in water went about 1/2 mile - stalled - wouldnt turn over - hydrolocked - water in those 4 cylinders.

To offer you a ray of hope, we've had two boats this year where the owners didn't cover the boats well - and neither had engine hatches on them. Believe it or not, both were hydrolocked from water running down the intakes and running into the rear cylinders due to the angle of the boat on the trailer. Would I have ever believed that just pulling the plugs and blowing the water out would get them running? No. It did in both cases. Learn something new every day. Go for the simplest stuff first.