agreed, definitions are important.
in the context of what i wrote, by self rescue, i meant the normal crew could recover from a swamping or a capsize without outside assistance and get the craft sailing or moving again and return home.
so with respect to swamping, a 'conceptual' open hulled boat (whose only buyoancy is created by the hull form), will sink.
in order to be self recoverable, you either have to close off sections of the hull or add properly secured flotation (which is equivelent to closing off sections of the hull), so the boat can be swamped but still maintain enough buoyancy to stay on its lines/maintain stability and if required, to be bailed without reswamping, and be sailed away.
so as old yeller recognized, there is a lot of space between sinking and self recoverable.
surfboads, paddle boards, windsurfers are probably the best examples (absent a hull breach) of 'closed hull' boats that are self recoverable.
lasers and sunfish are other concrete examples of boats whose hulls are primarily closed and are designed to be routinely self-recoverable from being swamped.
dyer dhows and penguins (and a simple rowboat or canoe) are concrete examples of boats that are primarily open hulled and while they often have flotation and won't sink, are incredibly difficult to unswamp without outside assistance once that has occurred.
it's hard to talk about 'day sailors' in the same terms as open or closed hulled boats. 'day sailor' is really a term about the expected use of the boat....and not a desciption of the design/approach to maintaining buoyancy. based on what i routinely see on the water, most popular day sailors/dinghys today are primarily biased towards closed hull desgin attributes.
that said, old yeller has it right when he suggests taking the boat to a safe place and actually giving a swamping/capsizing a go. that has been a pretty routine and enlightening experience in my dinghy racing background and is often part of the beginning of the racing season exercises (for both sailors and rescue crews).
its pretty interesting to see what can go wrong that can prevent you from rescuing yourself. everything from the mundane, like forgetting to secure dagger boards or bailers or flotation properly, to the unexpected, like a lifejacket with buckles that hangs up on the deck as you try to squirm back on board, to simply not having the upper body strength or understanding of technique required to re-board and empty out the boat.