With the boat in the water start the engine, then remove the incoming water hose from the thermostat housing. Try to stop the water flow with your hand. If it stops easily then the problem is in the drive, one of the grommets that seal the water pipe are leaking. Also if the pressure is more than you can hold back, then as posted above either the sender or the gauge.
To check for a combustion leak add a short piece of clear hose just before the incoming water connects to the thermostat housing. If there is a combustion leak you will see the water disappear in the hose, the combustion back pressure will overcome the impeller output.