Maybe he had a phobia about swimming that went away once he realized he was doomed anyway.
my theory on this is similar but worded a little different... i agre but what about this?:
he knows how to swim... but would have died saving claire because of the horrible undertoe... BUT he does have a fear of swimming... which was shown as a child... BUT when the change to save claire and turnip head .... a man in love would do ANYTHING to save they love... i would...