Is the water milky or more clear, dont some of the head bolts go into the water jackets ?? these gen VI ? wonder if thread sealant was used......

Water is pretty clear. Split the filter and it’s very Milky.

if its a 454,then yes,bolts go into water jacket,,502 are blind

Did it have captains call before or just through hull? Was the old exhaust dry to the tips?

It has always had captains call.

1622 is the last four digits of the stock GM cam??? 454/502 Mag Camshaft GM # 12551622 - Offshoreonly.com

I'd pull one of the cams and have the specs checked to verify. Stock exhaust with diverters can't handle any more than stock cams.
Hard to tell from the pics, but those corsa diverters don't look to have much downward angle.
Are they doing it when ran with the boat in the water?? or on a hose on the trailer??
And are the engines getting to full operating temp @160*+?? Stock oil coolers??
Post pics of the spark plugs, heads, and pistons.

Mistake

That sounds like two completely separate problems ? Is there actually "water" in the valley or is it milky oil ?

Did you ever figure this out?