If the engine is going 5 miles or less the warnining light comes on so l have to shut off the engine and restart it. It has been looked at by a marine mechanic and he cannot find the problem.Appreciate any help.

I've had this problem with my boat, and never could find anything wrong. But when l went to replace the water pump impeller for routine maintenance, about half of it was missing. Evidently, l must have sucked up some debris somewhere. I'd at least give that a check, but that will require dropping the lower end, replacing seals and oil and all. But it might be worth it in the long run.

ihad to replace my water pump .the impellers were shot,cost was cheap enough,dotn let the motor run without being in water on hose running on full to clean

Gotta go with the other two you have and change the impeller. If the impeller is changed and still heating up l would guess a blockage in the cooling system somewhere.

Have a temp test done to eliminate the possibility of a faulty temp sender. l presume your mechanic checked the impeller and pump output. Make sure you have good water pressure at the top of the block. This can be done by removing the tell-tail hose from the fitting on the block. If you have good pressure and a good pump, check that the water pipe in the midsection is seated correctly in the seal in the block. Good luck.

look for a loose electrical connection at sensor

theres 3 things to check[you didnt say what year the motor is] l have the same motor & had the same problem
1st change the water impellor if u didnt do it in the last 5 years,wheni changed mine theres a plastic tube that connects the copper tube to the top & the plastic tube was missing
2nd change ur thermostats u have 2 of them
3rd change the popit valve its located on the engine block
that should do it
if it still over heats change the sending unit or have it checked
actually do the 2nd one first andthe first one secound