Hi,

Had the boat several years with the water heater bypassed since I purchased.  We never really needed it (our previous boats did not have heaters) and the PO had jammed an A/C unit down into that compartment, which made accessing it frustrating enough that I never bothered.  Anyway, decided this season why not get this thing up and running again.  I removed the bypass, reconnected the water heater and filled the tank.  So I have a few questions and maybe just need to confirm my basic understanding of the system. 
With the electric heating element on shore power in about 30 minutes the water was scalding hot coming out of the taps - but with enough cold water mix it was fine for a shower. 
I went over to a neighbors boat for a few hours (with the heater left on) and when I returned the fresh water tank had completely drained.  After my shower and then my wife and bleeding air after reconnecting I assume it maybe had 5 - 10 of the 22 gallons left in the tank when we left for the neighbors.  I refilled the fresh water tank and turned the faucet back on and pretty soon found hot water streaming out of the PRV hose into the bilge.  After I turned the water heater off this stopped, so it seems obvious that is why the tank drained.  However, I guess I'm still confused after reading some of the other posts on here; I understand the PRV was doing its job by relieving pressure in the tank, but why does the thermostat allow it to get so hot that it needs to?  Supposedly pressure was 150psi or temp 210F according the valve.  Second question is, does this mean every time I cruise for several hours under power the PRV is going to release water into the bilge and subsequently be refilled by the pump until the fresh tank just empties again?  I've read some conflicting views on the temperature of the coolant that reaches the hot water tank - the engine always runs in the operating range at about 170F, but I don't know how that translates to its ability to heat the water in the tank.  I guess the question becomes, is something (thermostat, PRV, high limit switch) malfunctioning and how do I determine what.  I didn't have a lot of time to troubleshoot as it was late on Sunday, but I plan to return to the boat and take a reading of the water temp coming out the tap with the heater on again.