A quick google shows people estimating 500-750 kcal/hour when swimming the English Channel. And another big range, but it looks like ~15 hours is a decent estimate for a one-way crossing, so that’s ~7,000-11,000 kcal, which assuming 3500kcal/lb of fat, that’s 2-3 lbs.

This is all ignoring the fact that most of the energy burned is actually glycogen and food consumed during the swim, not fat reserves.