I remember reading an article in Pilot a few years ago about a guy who ditched a Cherokee 6 in the Atlantic. The tail was poking out of the water for a few minutes but the cabin had sumberged almost instantly.
When we go offshore, survival suits are mandatory. I agree with the comments about a dinghy being hard to get into. It's bad enough in a swimming pool, in a North Sea swell it can be very difficult even with others helping you.