Pushing the old brain cells here but I do have pape record of a 1996 Cabo 35 convertible with two fuel tanks for a total capacity of 425 gallons and one water tank of 110 gallon.
I then have a record of a 2001 Cabo 35 Convertible with a single fuel tank of 400 gallons and a single water tank of 100 gallons.
I do have a photograth showing the two battery boxes between the main engines just inboard of the forward engine mounts which would go along with the above photo graphgraph showing hull # 1 being built.
I also have a photo graph showing blue fuel hoses going forward and up into what appears to be a fuel tank above the forward fresh water tank.
I believe the battery charger is forward to starboard with the water pump and the water heater aft of that all to starbaord and accessed by lifting the salon deck hatch inboard of the starboard sofa and forward of the starboard engine hatch which is below the aft starboard seat.
The air conditioning unit(s) and sea water pump are to port and accessed through the deck hatch inboard of the galley.
I think the box you are thinking of C. J. are on the later model 40 & above boats where you have 5 or sometimes six group 27 batteries in a vented box forward on the centerline with a bilge pump below them behind another access hatch.
Anyway, try tracing the water lines from the starbaord mounted fresh water pump to the tank.