I'll bet, because of the U.S. boats, they had the highest market share at the time that you have a 6 gallon Rairitan heater (now the 1700 model).

I'd blow out the remaining water where you are and through a little antifreeze in there. They would suggest that too from their manual (which you can find on their's and other sites).

I don't know if the heat exchanger has any coil in the bottom which might be impacted by by freezing or whatever.

This is an excerpt from the Raritan site:

WINTERIZING
Before beginning winterizing, be sure the power is
turned off to the water heater. Open all hot water taps
on the boat to relieve the pressure and allow cold
water to cool the water in the system. Remove both
lines from the heater and allow it to drain. After it
has drained, blow it out with air pressure, if available.
Pour a quart of nontoxic antifreeze (propylene glycol)
into the hot water fitting of the tank using a funnel
and short length of hose. Be sure power remains
off until spring.
When recommissioning in the spring, run water long
enough to insure that all antifreeze has been flushed
out and tank is filled BEFORE TURNING ON
POWER. Heat exchangers are winterized in
conjunction with the boat’s engine cooling system.