My best advice is to ditch the Seward water heater, and get a Bosch 2.5 gallon. Put that under the galley and it is close to points of use. It solves the issue of water waste by the long Ines with the water heater on the other side of the boat. There are several threads on this. My album and others have photos of the Bosch in place.
The bonus is that you get a great increase of storage where the water heater lived.
If. you want to use the water heater, flush it with several runs of fresh water thru the tank. You want to clean it. Then turn it on (1200 to 1500 watts to be sure it heats. It is possible that you may have to replace the Anode in the water heater. It will develop a sulfur odor with decay and stagnate water from non use. Be sure that the bypass goes into the shower sump or overboard. you should be able to drain the water heater and see if there are de[posits there.
