Treating interior plywood in a small boat

[Boatwright]

Nothing wrong with exterior rated regular ply for a non-structural application like seats. Most plywood sold these days has exterior rated glue. Stay away from treated ply. The preservative salts actually attract moisture and the stuff also tends to warp badly.

I've been using Rustoleum enamel for interior boat paint for decades. It is a high-quality alkyd (not the same as acrylic) oil-based enamel, reasonably priced and available everywhere. Lt. grey is the best choice - covers better, hides the inevitable dirt, and is the classic color for boat bilges, and hidden wood such as your seats. Follow the manufacturer's instructions re thinner and thin about 10% for good penetration. You do not need multiple heavy coats - one on the underside and two on top is plenty. The idea is to seal and stabilize the grain of the wood, while still allowing the wood to breath. Brush application is fine.

Regular epoxy resin is a bad idea because it traps moisture. 30 years ago dozens of mahogany runabouts (Chris-Crafts) were restored using West System epoxy as a sealer which was supposed to permanently protect the wood. It didn't and sadly many of those boats quickly rotted out. There is one epoxy product: Smith's Penetrating Epoxy Sealer that is very thin and works well as a sealer esp. under fancy varnished brightwork, but it is very expensive and very toxic.

Keep it simple. You can spend the money you save on rigging, etc.. Have fun and a welcome from one of those who find nothing quite as fun as messing around in boats.