Water heater questions

[Bottom Line]

I have a 6 gallon Seaward water heater on our 360DA. Our first night on the boat, I noticed the fresh water pump kept cycling overnight. The next morning I found the water heater was periodically discharging overboard. Yesterday, I replaced the T & P valve. A couple of hours later, I noticed the water heater was discharging overboard again....so I am guessing the water is getting hotter than 210 degrees and the valve is releasing the water. There are 2 other parts that I have noticed in the owners manual, but not sure how to isolate which one to replace.
One is called a high temp limit switch and the other one is called a fixed temp control thermostat. Does anyone have know which part may be allowing the water to get too hot? Thanks for the input!

 

[northern]

I have an Attwood tank. Mine went like yours
This is what you need $10. If not an Attwood others probably work the same.
http://www.dyersonline.com/atwood-w...tat-eco-assembly-for-pilot-water-heaters.html

If you tank is older than 10 years replace it as it will fail soon