Originally Posted by InfernoST
If it were me I would have put 2x3 in all of the corners then glued and screwed into that then glassed the the whole thing.
Yep - me to. Those butt jointed corners with sheetrock screws holding the plywood together are gonna be the weak point.
I hope it works out fine and your build can sustain the weight and pressure from this much water. However, it's not to late at this point in your build to add structural integrity to each of the four corner joints.
You mention buying the best plywood available - - was it a structural plywood or maybe a marine grade ply or just the best grade of exterior plywood available at your lumber store? Did you notice what the core value was?