You might want to do a search on this topic. Lord knows it has been on this forum before. Some of the culprits could be the incoming water hose that runs from the drive to the transom. Where the water neck bolts to the transom on the inside, it sometimes will crush from the salt build up and restrict water flow. If the boat has a Fountain flush system on it, make sure the valves are in the correct position and open all the way, as well as the flush cap is installed. Did anyone ever have the drive off and forget to put the o ring on the drive around the water pocket? Does the boat float, does it have groath in the water pickups in the drive? When it over heats does the alarm go off? If not it may be a faulty gauge or sending unit? I have even seen where the coating comes loose inside the Gil manifolds and will restrict water flow at an outlet from the manifold to the riser. The water pressure gauges on a Fountain I believe are on the side of the block. I have also seen some on the exhaust manifolds at the back. Check all of this out as well as what others are going to tell ya about and let us know what you find.