If your marina is on well water it could be a problem with the water treatment system that's been fixed (you said the second tank was OK).
It's probably not the anode in your water heater or the smell would have continued with the second tank.
I shock my system every spring. I pour about a half cup of chlorine bleach in each tank, fill them with fresh water, pump them dry, then repeat with fresh water again. The first time you do this make sure you take the screens out of your faucets as you'll loosen up a lot of gunk.
All water going into my tanks or into my dockside water hookup goes through a ShurFlo filter with a hose attachment. They cost about $25 at the Camping World
or $45 at WM. Replace it each spring when you commission the boat. When I fill the tanks I drop in a capfull of bleach in each tank (50 gals total).
With this approach the water always tastes and smells good and we've never had any problems with contamination.