ian is spot on with his advice, i had a transit all ply lined,i hadnt relised just how much water was being held under the ply floor,i was going to protectakote it and lifted up the plywood floor to get it ready,but the floor of my transit was so badly rotted i had to scrap it,even with welding it would never be safe to carry heavy tank of water again.
as Ian said if pure water is being held under ply or rubber floor you will not belive how fast it will rot your van.now driveing a movano and ply floor was ripped out asap.