When I replaced the water heater on my boat (about 7 years ago), I lifted the coolant hoses above the engine, and gravity kept the coolant in place. After installing the WH in the boat, I attached the hoses to the new tank and added some antifreeze to top the unit up.
I'm not sure about your unit, but once I released the upper of the two hoses, the liquid flowed out of the other hose towards the engine to find equilibrium. It was one of the easier projects on my boat.