Steve, the rotten smell indicates that the water in the water heater has not been used for many months . There is no anode installed on these water heaters. The manufacturer will provide one if you like at an additional cost, but you will have to remove the drain valve and install the anode in this opening. I would rather keep the valve.
You can clean the water that is in the water heater and restore the water heater. Drain the water using the drain valve. Disconnect the electrical wires, remove the heating element and drain more water. You may see more muddy water coming out. Reinstall the heating element and fill the water heater with 2 Oz of chlorox premixed with 5 gallons of water then drain the water. Repeat 3 times. Then repeat with fresh water 3 times.
The smell will be gone.
Whenever I am at the boat, at least twice a month, I drain one gallon of hot water to make sure fresh water has entered the water heater. Whenever I fill the water tanks I add 1 Oz of chlorox to the tank to help kill the bacteria.

Actually you can replace the drain plug with an anode attached to for about $20. It seems some boats do have anodes and others don't according to Sea Ward, the manufacturer of my  tank. That way you still have the drain function

I bought one but have not tried to install yet. Seems odd that it's a "you may or not have an anode" situation, but that's what it is.

After posting my last I found the contact info in an old email:

 The anode is optional, so there is not necessarily one installed. It replaces the drain-valve. I stock this version by Camco which has an integrated drain so you don't lose that function if you wanted to replace/add one:

    https://www.suremarineservice.com/Galley/Kuuma-Replacement-Water-Heater-Parts/11916.html

Cordially,
Graham Milgate
Sales and Service
Sure Marine Service Inc.
5320 28th Ave NW
Seattle, WA 98107
Shop: (206)784-9903
Fax: (206)784-0506
Direct: (206)731-7848