Ok, I got the boat on the water with the new impeller and thermostat and had the same problem. I checked everything and the only thing I can come up with is the water pump/circulating pump. I took off the big hose that goes from the pump to the thermostat housing and had my wife start the engine. No water at all came out of the hose even thought there was still water in it. I assume that that has to be the problem. So I will tackle that next. I'll let you know what I find.