WFP tank is a modification to the vehicle. If you dont tell the insurance company, then should you be in an accident (your fault or not) they do not have to pay out.
Carraige of goods would apply to a non bolted down tank, but there is a possibility that they could argue that one out seen as though water is carried on a regular basis.
I dont know my insurer as my wife sorted it all out for me (im a busy man) oh and they payed out when i backed the metal bumper into my mother in laws merc slk whilst she was watching (shouldnt have parked on my drive)
Not a clue what i pay and modifications declared