You know, re-reading your post, do you mean the water tank that is under the port aft seat? It's kind of hard to tell from your post. If so, I don't know of a direct replacement, but I found a number of tanks that would fit just fine. Varying numbers of gallons vs. how much space you end up with above the tank.
My original tank (which measured out at 17-18 gallons - maybe the exterior dimensions equal 20 gallons...) was not a great fit anyway as installed (outboard edge was riding on chine; bottom was not supported) (although that's solvable by installing a 1/2" base/board).
One good source is Ronco plastics - they make great tanks and have quite a few sizes. They'll put the fill/vent/supply where you want them.
Just for reference, my original water tank was 19" wide, 20-1/4" long (inboard to outboard dimension), and 12" high.