on his 800 gallon plywood tank build the fiberglass and resin inside and out has alot to do with the strength.
i tried something similar with a 300g and i didn't do the whole out side just the seems but i did fiberglass the whole inside, my tank bows alot.
liquid rubber water proofs well i used it to save my tank build because of the flexing, i plan on using liquid rubber in my next build but i am going to build a external frame.
i don't know how well liquid rubber will hold up with salt i would call the manufacture.
for the acrylic thickness i'm no expert but i think for 3' you need at least 1 to 1.5" i believe just because acrylic bows alot.
for sealing the window in you will need to do a fiber glass bridge between to silicone and liquid rubber or if your using acrylic you can go with a gasket method or i have seen a build on another form where he used the liquid rubber to seal in his acrylic window on a 2200 g build.