I'm not sure what the previous poster meant when he called Herculiner "fiberglass". I'm assuming he meant it's polyester which is the typical resin used to wet out fiberglass cloth, roving, matt, etc.
It is not polyester. It is a single component "rubberized" linear polyurethane in a xylene carrier.
It's about 75% solids by weight so when you're paying $175 for a gallon understand that you only end up with 3/4 gallon once the xylene evaporates. So that equates to more like $230 per gallon of solids. Pretty expensive compared to other choices availabler for plywood tanks.
Now with all that said, both single component and two component truck bed liners have been used to seal plywood tanks, although not all of the examples I have read about had viewing windows.
If you already have the product on hand I think there is a good chance it will work although I don't recall Herculiner being used specifically. But if it's similar to other 1k truck bed liners then it should work. But I wouldn't try to silicone glass directly to it unless you run a test sample first.
If you don't already have the Herculiner on hand then I would research other options as well.