A few years ago, there was a photo of a guy in a Macgregor 25 who had hit a floating hazard in New York harbor, and holed his boat. He was able to motor back into his point of origin, with about 12 inches of freeboard, due to the built in flotation.
The original version of the Pearson Ensign (3000 lb displacement) had no flotation, and several people were drowned when caught by bad weather while racing in San Francisco harbor. Subsequently, all Ensigns sold had foam flotation built in. Other similar sized boats having flotation (Air tanks or sealed compartments) include the Soling and the J-22. A friend left the flotation compartment on his Soling open, and had to hire a crane to retrieve his boat after it capsized and sank.
My first sailboat was a wooden 16 footer with a fixed keel and a 19 foot wooden mast. It was very easy to capsize and sank like a rock, but sat upright after sinking. I was careful not to sail in deep water so I could retrieve it if I capsized. Since then, I have stuck with boats having flotation.