I got SHOCKED Help!!

I have a 2004 Rinker 290 with twin alpha one gen II outdrives. This morning went outside to de-winterize and start them up. The boat is plugged into the house in my driveway and when I went to put the muffs on I felt a slight electrical zap on both outdrives. I unplugged the shorepower and it does not do it anymore. Is this normal? I have no electrical mercathode system just zincs.

Update!

Ok this is what I came up with. The new water heater was installed yesterday, before that no problems at all. I worked on the outdrives last week replacing the impellers and did not have any zaps. Installed this brand new seaward water heater yesterday and this morning got shocked from touching the outdrives. Here is a question I can't figure out. The main power to the water heater was switched off but I am still getting 65.8 volts at the connector. I turn the breaker on and I get 120 volts. I disconnect the water heater and I don't get zapped. I reconnect it and I get zapped. Should there be some voltage at that plug with the breaker to it turned off or is this a problem? Or did I get a bad water heater that has a grounding problem?
HELP!!!

without knowing the details of the boat wiring it is hard to help you....the last question you asked is a definite no on straight 115 house wiring...cut the house breaker off and leave everything plugged in...take a ohmmeter and you should read zero or close to it between neutral and ground if you unplug the heater receptacle and read back towards the source...i assume you have a ground wired to the heater end of the plug that goes into the receptacle that plugs in to connect the heater.....what i think you have is neutral and hot swapped somewhere on the boat...and if you got a generator that will complicate things...

do you have any 115 volt plugs on the boat that are two wire and ungrounded?you are getting feedback through some other device is my guess...its a guess because i dont know the complete wiring...

Open up the cover on the water heater and inspect the wiring connections. Of course you should have the power off.