Planked boats rely on the water to swell up the planks, preventing water from entering the boat. If a boat is allowed to dry out, the planks will create gaps that the barrier coat will not fill properly. Barrier coats will also hold in any moisture, promoting rot.
Plywood boat hulls and wooden boats that do not rely on expanding wood to seal out water can be barrier coated, but to prevent rot, they must be completely encapsulated in barrier coat on the top and bottom. Other wooden boat components can also be primed in this same manner, but should also be primed completely, preventing water migration to the wood, which inevitably leads to wood rot problems.