There's quite a difference in diving into a pool and retrieving a rubber ring at the bottom of the deep end and swimming in the ocean surf. Charlie may of felt he couldn't handle the tide coming in. Also he had quite a different motivation for going to the Looking Glass, he was saving Claire and Aaron and Des had already told him he could accomplish it.

That is a good point Ralf, when he swam down to the looking glass, he already "knew" he could do it. Plus he had the weight belt helping to pull him down, so it wasn`t really that far he had to swim.

plus I think he just told Jack he was a swim champion because Des told him he was the one that was supposed to go down to the Looking Glass...HE was the one that saves Claire and Aaron. I think Charlie lied so he could do something good in his life, something to be proud of....to be a hero like Nadia called him.

^^ yeah.