Several possibilities I think.
1. There was a floatation device in the water he could grab onto
2. He had something that would prevent him from drowning, but not keep him afloat (e.g. an oxygen tank)
3. He had a means to get back in the yacht other than a floatation device (e.g. a rope tied to the boat)
4. The "deep water" had a means of escape (e.g. a lagoon/lake where the bank is climbable, or an artificial reservoir with a ladder) - though this doesn't explain why he needed to be rescued.