3 mm ply and glass only on the keel and chines. Run a carborundum strip along the keel for crashing on to rocky beaches. Admittedly it does leave grooves in the rocks but no one has complained yet.
Note, no glass outside or in, just on the joins, in and out. Oldest kayak? Built in 1983, still used occasionally. Most recent? About 7 years ago. My partner's most recent, half built and when finished will weight about the 18 kg mark. The first one she built is closer to 24 kg and used heavy 4 mm ply and glass one joins. Built ~1999, paddled in places all over New Zealand from Stewart Island to the Bay of Island, top of the North Island. Admittedly she did change from varnish finish to a painted finish a few years ago. Ultra-violet and varnish don't agree.
Weight for my last two kayaks, finished, ready for sea - 18 kg and even then weight could be removed. The latest one built was 13 kg in the water for testing before final fitting out.