A number of things come to mind...
Your water line to the mixing elbow may have come adrift...
Water pump failed (possible overheating)
Heat exchanger plugged badly, as you suspect.
Not sure what you mean by "no exhaust either" - while engine was still running??? Inside the boat was hot, was it fumey too? Could you smell exhaust gases down there?Is the exhaust hose from the water lift to the outlet intact?
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)