The plywood sea kayaks are all single chine hulls, built in 3mm or 4mm thick plywood, with a 200 gsm fabric on the exterior recommended.
These kayaks are capable (in experienced hands) of extended open water passages. We know of Bass Strait crossings having been done in some of these designs.
It is vital that anyone contemplated using a sea kayak that they learn how to exit from the kayak in the event of a capsize. A person close to us died when a sea kayak capsized (not one of these designs!) and he was unable to exit the kayak.