Re: sealed enough?
Meh, considering that manufacturers don't even seal the plywood on new boats I'd say that wi's job is better than factory. I've also seen a few resto threads where they just use 2 coats of urethane sealer... and 2 coats of epoxy is much stronger than 2 coats of urethane.
Rivets and screws fill the void into which they're inserted and unless they constantly submerged the amount of intrusion is minimal.
With that being said, I personally would buy another 1/2 gallon of epoxy resin and apply 2 more coats to the top side and edges because I'm OCD. The bottom of the decking isn't going to get wet unless it's in constant contact with wet foam... or the boat sinks.
It's one of those things where it depends on how long it needs (or the owner desires it) to last. These are the same types of methods used to make these boats years ago, 90% right but lacking in a couple areas, this resulted in premature rot. Drill a hole for a screw or something and don't seal it and it will be the entry point for water.
I tend to recommend two routes to go, an owner can do everything possible to prevent future rot, even using no wood, or do nothing to prevent rot and get it done cheap and fast. Even doing nothing to seal the plywood will easily result in 5 to 10 years of service with no issues and sometimes even much longer if the boat is shown some kindness in life.
If someone wants to actually prevent rot for as long as possible you can't take short cuts, every detail counts, one shortcut and you may have undermined the entire effort. So if 3 coats of epoxy is needed for a waterproof finish (nothing is actually waterproof) and 2 are applied, it means it may fail sooner, maybe much sooner. The difference in lifespan between an uncoated surface and one with too little product applied may not be much, so the effort and money may be wasted.
Spar varnish and some other methods and mixes sort of work, but they aren't really that water proof, more like very water resistant and can have a limited lifespan, most need to be reapplied frequently to prevent damage.
When any of these products are applied under a carpet they are stressed to the max, it's about the most hostile environment for any product to survive in, so it needs to be done right if you expect it to last.
