Try to run boat with raw water intake hose in a bucket full of water NOT using a hose to feed it. If the engine does not SUCK through this hose (and quick!) then something is wrong.
If the mechanic is "pushing" water through the engine with a hose, this is your problem. My money is on the raw-water pump being backwards or the impeller being roasted. And the raw water pump is NEW? Not just serviced?
Also -- pull the thermostat out. No reason to even have it in there until you get the engine running cool.
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.