my dad and I made a kayak using marine grade plywood several years back. It has multiple laminations and the glue is waterproof. It also doesn't have gaps in it like normal plywood.
In addition you could line your corners and seams with fiberglass tape and then layer them with epoxy, then epoxy the ******** of the lagoon. This would probably work without a liner, or could be used with the liner as a safeguard in the event your liner ever punctures