Most boats have a water Deck Fill fitting like this, but read what it says Water, Fuel, Waste, whatever. In many cases you need a key or tool to open. Good Luck
If would help if you had bother telling us what model and year boat you have …. generally speaking look for a filler cap marked water.
Let me use my special, all knowing basket-ball to figure this out. Crystal-ball has been off-line for a while now. Ah, It's a Carver owner on a Carver boat, if the OP is correct, a female.. My special basket-ball explains more but not repeatable on this thread. To help the OP, walk around the toe rails and look for a water fill. Nothing? Then look on the transom, one of the upper corners. Oh sheet, update; On some Carvers there was a water fill on one transom corner, fuel fill on the other corner. Don't do what I have witnessed , read the fill plate.
