I cannot swim properly but I got through the VAA training no problem.
You have to be able to able to swim backwards whilst wearing a life jacket - difficult for anyone I would have thought. Make friends with a strong swimmer on your training course and buddy up for the practical.
Being able to swim could save your life, but in reality (and in 40ft waves in the Atlantic) there are probably more important aspects of your ability and skills that will keep you alive during a ditching!
Good luck. xxx
PS Not all airlines require that you are able to swim, although they all assess a ditching practical. Enough said.