Generally a check valve should NOT be installed on the cold water
inlet to the water heater. Least wise without a thermo expansion tank. For the typical marine
water heater with side hot and cold connects, thermosiphon is minimum.
But when heating
water in the tank, the water does expand. Putting a check valve on the cold water inlet would prevent that and cause extra stress on the tank and downstream. The plumbing
codes would not approve that without an expansion tank. Of course that does not apply to boats, but its still a very good idea.
On the diagram previously it showed a check valve after the accumulator. In reality the check valve should be BEFORE the accumulator. This as the accumulator will also act as a thermal expansion tank.
So generally there is no real reason to install a check valve on a water heater. 30 years ago it was common practice to install a check valve with a 1/8" hole drilled in the clapper at the inlet to the water heater. The hole was to allow relief of thermal expansion. That practice was changed 20 years ago.
Sailorchic34, PE, CIPE