You want to somehow route the fresh water supply around the tank so you don't need to fill the tank with antifreeze, and then drain the tank.
On my tank setup, there are two valves, one on the house water in hose and one on the house water out hose. Closing both these valves shunts the house water into a bypass hose so there is no house water going into the hot water tank. If you don't have a shunt like this, you can disconnect both hoses from the tank, and connect themn together for the same result. There should be a drain spigot. Open the spigot to empty all house water out of the tank. Close the spigot when empty. You are done.
When commissioning the boat, open both valves or re-attach the hoses.