Russ, If it's under the dining seat forward of the galley, as mine is on the Hunter 33.5, you have to remove the entire dining seat assembly as one unit. The trick is locating ALL of the screws. One or more screws were in the cubby next to the hull. After all screws are removed, the unit just lifts out and you can stand ti on end in the main salon aisle. There are only a few screws (about 4) holding the water heater down to the plywood sub deck. If it didn't have a heat exchanger in the old heater, you will have to run two water (heavy wall) hoses, one from engine cooling to the heater and one return from the heater back to the engine. Study the best route for these 2 hoses. Of course you have to disconnect the pluming water in and hot water out lines.
Dick, S/v Puffin, 1990 H-33.5