Just turn off the sea cock and pour it into the weed filter then turn the engine over till it comes out of the exhaust, topping up as you turning it over
And take gas water heaters apart and drain them completely and then take them home because they are swine to empty and not leave residue to freeze and break the exchanger.
And if you have a shower make sure the temp cassette in the control valve is emptied and blown out, unless you only have a caravan type in which case just drain everything down and blow the pipes clear.
Mind you - I might be too late with that advice now.