Most plywoods are very moisture resistant (interior and exterior). Many will stand up to full submersion, dry cycles into the 100s of times. Melamine is not water resistant at all, and needs to be fully encapsulated every joint sealed before gluing and after, no water can get in and no gas should escape. It is used because it is cheap, the same reason why it is used for cheap furniture.

Yes plywood is best sealed up it looks better and is easier to clean it prevents stains it is not however about a moisture barrier. This is why it is fine to use water base products (permeable) The toughest coatings and most complicated are two part LPs and epoxies they are close to full barriers. One part polyurethane is the hardest of the one part coatings (varithane for example) Next would be oil based coatings they often are very durable deck products and bathroom products should be avoided (often contain toxins to prevent mould or insects.) The same can be said for 'spar' varnishes they typically are soft to be used on flexible wood (boat spars! AKA masts ect.) they never fully harden and wear poorly. There are a handful of hybrid coatings modified latex ect. they can be hard wearing as well. Then the true latex water based coatings are last they are not as hard wearing as the others but offer easier use and lower toxic compounds when working with them. All coatings need enough time to fully cure and out gas. Most oil based can be baked out with heat and lots of air flow epoxy and LP need at least a week in the correct temp for that product (huge variations) if you have never worked with LP or epoxies they can be very tricky and need some practice (and cost lots so it is hard to get experience).