I need to thank Falco - I had many of the questions he brought up. I will give my reasoning - probably faulty - for the choices I made in connecting the water heater. First my understanding of marine electrical systems is very limited. I bought 'Marine Electrical and Electronics Bible' which I found difficult to use but got some general concepts.
1. GFI plug. I know you are correct. I had an old plug in my tool box which I used. All of my other AC outlets are connected to a GFI plug. But, faulty thinking, I thought of GFI plugs primarily for appliances you handle, like hair dryers or toasters. As I understand the GFI, it responds to a current surge just like a breaker box but far more rapildy. Will replace the regular plug.
2. Bonding. Here I am really confused. I thought bonding was not to protect us humans but to protect multiple thru-hull inlets/outlets from metal loss. Also I thought this pertained only to DC systems. The purpose of bonding was to make sure that all of the thru-hulls (or other metals) that contacted the salt water interface had identical electrical potential. If there was no bonding, and the thru-hulls had different potentials, then metal ions would flow from one to the other to even the potential and result in destruction of one of the brass or bronze thru-hulls. So I really need an education. The water heater is grounded through shore power and is protected with a breaker. Also there is a magnesium anode in the water heater which I assume is protective of galvanic current.
3. Check Valve: What you say makes sense. However the following was in the water heater manual: "The water utility supply meter may contain a check valve, back flow preventer......... that will create a closed water system, During the heating cycle of the water heater, the water expands causing pressure inside the water heater to increase. The temperature and pressure relief valve may discharge hot water under these conditions which results in a loss of energy and a build up of lime on the relief valve seat...."
It says that if you have a check valve you need either an expansion tank or a pressure relief valve in the cold water supply line. If the system seem to do well without the check valve should I leave well-enough alone.
Again, thanks to all who checked-out my hot water system. I would appreciate more input on the questions raised by Falco.