Swimming 9 miles is tough because of how long you are in the water (period). No drinking water, so you are dehydrating for 8-10 hours if you swim well, longer if you don't (how long was this guy in the water?).

I used to swim a mile on my lunch break, then I had lunch. The only swim strokes I use are distance strokes -- breast-stroke, side-stroke. I did a 45 min mile using a breast-stroke, but that was for all of 1 mile in a pool. 9 miles in the ocean is a long distance to just pick up after finding yourself unexpectedly in the water.

Necessity is a great motivator; he had no other good options.