Working on a friends boat, twin engine 525SC's on header started to overheat, turned yellow in the collector area, burnt the rubber hose that connected tip. thought silent choice stuck closed and ran hard, But I and CMI thing lack of water is the issue, just seeing if anybody else has heard of this...replaced the flapper for silent choice, tip, rubber and ran again burned hose and cracked at weld on forward tube...

Sounds like a flow issue to the header.

Lack of cooling water.

what I think to, just have not found anything, water flow from exhaust seem equal at low RPM, but whatever is happening is while running hard

check the header manifold for blockage, those hoses from the T-stat housing to the header manifold aren't reinforced and can collapse from inside.

Or possibly the exhaust tube is cracked and letting exhaust into the water jacket.