You know, you buy a boat, and there are going to be a few things that you and the surveyor are going to miss. Well, I have to tell you that this water tank was broken for YEARS. Somebody tried to stop the leak by just pouring epoxy down the cracks. Hello... When I put water in it, it slowly dripped out and soaked the cockpit sole and made my life hell. The real culprit was the fitting, which was just plain cracked.
The only way to fix it was to take my trusty Dremel tool and route out the top of the bench, pull the tank and figure it out. The solution was a new tank. Fortunately, a number was stamped on the old one, and Bell Plastics in Hayward CA was able to find an exact replacment.
I had them weld all the fittings into the TOP of the tank! This tank will never leak again! But if it does, I painted the compartment with epoxy and fiberglassed the seams, and installed a drain plug just in case. When I installed the tank, I braced it with wood and stuffed closed cell foam to eliminate chafe.
The outputs fittings (with filters) are at the top, one for the the galley and one for the head. You can easily check the state of the filters, and replace them if necessary. Bell Plastics installed a 4" plug, but it wasn't right for a marine application, so I installed a 4" deck port inside the fitting, sealing it carefully with silicone.
The port needed to be 4" so a normal person could reach inside. You see, with the output fittings at the top of the tank, I had to weld threaded fittings on the inside of the tank as well (cost extra, but...)
Attached to the inside fittings is a length of flexible irrigation pipe, which reaches down to the bottom to draw the water up and through.
This works great, and the cabin sole will never flood from the water tank again.