Hope you find where the water is getting in or out and can stop it. Good idea for the marine ply.
Looks like a good project and hope it goes smoothly. Be sure and take some pics and post so we can see how it goes.
WELL! Back in Oct. I had big plans to remove fresh water tank to replace entire wet bay basement floor.
HA HA HA!
Turns out I could only move it 14” toward passenger side. That gave me enough room to use an idea suggested and fiberglass weak portion.
Underneath I bolted 3/4” treated plywood to metal frame using 5/16” bolts and 1-1/4 fender washers top & bottom. (Some tubes in pix are drains-some are pieces of tube covering SS bolts protecting threads from undercoating.
I keep a 200 watt heater on each side of tank plugged into a t-block. Has kept tank and lines from freezing for years.
A little more caulking underneath & cleanup on rocker panel & it’ll last a few more years. Peter