Our current boat has a tankless, on demand propane water heater. But, I remember all too clearly, having the need to winterize a water heater like this for many years with past boats.
Sailingfool's explanation was very concise and spot-on. You may find however, that your in
hoses lack the cocks his convenient system has and will need to connect the two hose ends after draining the tank.
My hoses had NPT threaded female fittings that conected to the tank's male threaded nipples. What I used was a 10" length (your length may vary
) of bronze pipe, threaded at each end, to connect the two hose ends. Use teflon pipe thread tape to seal all connections.
This enabled pressurizing the system after draining, so a couple gallons of non-toxic pink anti-freeze could be pumped through. Be sure to closeguard those tiny bronze drain plugs - they are easily lost to the bilge's black hole . . . speaking from experience.
BTW, It's amazing how many boaters make the mistake of referring to their water heater as a "hot water heater".