[Faster]

If the old tank is rusty and disused, there's no reason not to remove it.. And as PB says electrically it's no big deal.. if your boat's unplugged from shore power when you do it the circuit is sure to be dead. Once disconnected, be sure to disconnect the wires at the breaker too, so that you won't end up with live wires when you're plugged in.

If you find yourself spending more time away from docks and want hot water again, then look into buying a good SS tank and install it into the coolant loop of your engine (if it's FWC) and you'll have the best of both worlds if you reconnect the electric element. It is a big plus for resale.