This is a good time to remind the OP that all marine
plywood is not created equal; in fact there can be big difference between various manufacturers of marine
Presumably the OP is North American based and while I'm not familiar with their suppliers I suspect it is similar to the Australian market.
Here the two main specifications are the British standard BS 1088 and the Australian standard AS2272. While the Australian standard produces a far superior product to that of the BS 1088, it is (again presumably) not available in North America. So the OP is left with getting the best product he can under BS 1088 or any North American standard.
Be aware that the BS 1088 stamp is sometimes forged or otherwise not meet by some less scrupulous manufacturers.
All genuine marine ply will a type A bond (waterproof glue), both face veneers free of defects, all veneers to be a "rot resistance" species and less than 5% voids in internal veneers.
The better plys will have a single
species used on all veneers and way less than 5% voids on internal veneers (and minimal other defects). While price
can be a guide, it is of course, no guarantee of quality except to say, the best marine ply is never cheap
For a fully submerged reasonably sized rudder, I would be sourcing the best quality ply I could in my area and use normal epoxy techniques. As others have posted, the glass is really only abrasion protection and as a medium to ensure a decent thickness of epoxy.
Edit: Although epoxy is the best coating, these days I would be happy to use other products for gluing during the construction phase e.g. a waterproof expanding polyurethane
or the old (& unbeatable), resorcinol - if your wood working skill are good enough.
allows for quick(ish) curing and is very tolerant of voids when fitting the components together however it must be well clamped during it's short curing phase.
I doubt there is any over the counter product that is stronger or more waterproof than resorcinol but it is completely intolerant of voids and also must be well clamped.
Of course, epoxy is great for strength and waterproofing, reasonably tolerant of voids and should only be lightly clamped. The downside is the longer curing times, hard to work when cured and the general mess