Hi Stuart,
I have no experience of these, if you have the battery power available and the correct size stranded cable to supply this immersion heater, then it should be fine.
It will draw 25 amps at 12 volt so a minimum of 2.5 mm2 cable is required for a short run, longer I would go for 4mm and it will need a 30 amp breaker to control it.
Regards
Alan