Is subdivision of a hull with watertight bulkheads satisfactory? It was another alternative I was looking into before realising the foam core was enough.

The main bulkhead under the forward beam was going to be watertight with a bung at the bottom to enable drainage after sailing. This would make two compartments in each hull with about 150-200L each.

Watertight compartments are a common feature in dinghies and are compulsory on larger commercial vessels.