I bypass the heater and drain it as do most reputable yards up this way. Flushing the tank of PG in the spring is a royal PITA. I use a drill pump to suck the water out and then a 3" brass nipple for the bypass because I used flexible stainless hose off the ends of the PEX tube that feeds the water heater. On some boats you'll need to figure out what fittings you'd need to bypass it. If your heater is in an area that it runs direct to a good bilge sump then just opening the drain and allowing air in through the HW supply outlet will work too.

Here's a different heater and bypass loop.

Blowing air into the hot water supply inlet will help to force any remaining water out of the tank. On most tanks some water will always remain because the drain tappings are not at the exact bottom of the tank. Even though not 100% drained there will not be enough water remaining to cause it to split.