In a 400 pound tank like a SXR. The water box isn't the problem. Kawi over built that pig. You can grind 20 pounds to extra bond line sealer out of most of those hull.
Wamaltons makes a composit box that is unreal light for the sj. It will work in a SXR hull but that's not your biggest problem.
20lb is a bit over estimated. I took ALL of the extra glue from the bond line area in the front and rear, around the bulkhead and around the water fittings, put it all in the same box and had 11.33lbs. Every ski will be a little different, there was a HUGE glob of it up front on mine. I did not include the giant sheet of glue that held the tray to the pump tunnel. It was a crap load of glue, but I didn't consider it "extra" glue since it had a function, but my tray will be glassed to the pump tunnel when it gets put back together. That chunk was probably a couple pounds by itself.
The biggest factor in the SXR weight is how thick the hull is. The bottom hull is .58" thick on the very bottom and it was done in layers. They did like .25" of glass, then added the bed plate, battery plate, plate under the tool kit and hull inserts, then added another .25". Crazy thick. I split the top and bottom decks and my bottom deck with the bulkhead and nothing else was 74lbs...same weight as a carbon BOB with hood, hoses and fittings, ready to put together.
I'm putting an ATP box in mine, way lighter to start with, and less volume, so lighter while running and full of water as well.
