Unsure about the legality side of it - but I would suspect it is. As it is a requirement to have a drill for an unpremeditated ditching in the SEP manual, which would be a catastrophic event then I would consider it to be also a requirement that every crew member can swim. In this situation you would not already be wearing a lifejacket as in a premeditated emergency. Your subsequent actions, when the aircraft came to a complete stop would be an immediate evacuation which you would aim to have started instantly that it was safe to do so. You would probably command the passengers to grab their lifejacket and get out. On completing the evacuation, you would be required outside the aircraft to do, or adapt what you have been trained, according to the manual, as quickly as possible. If you could not swim, then you would not be able to put on your orange life jacket in the water, hence would not be able to carry out your safety related duties adequately. This probably means that your airline is employing people who could hinder the chances of survival in the event of a ditching, hence they are not taking every step to ensure their passengers travel as safely as possible. (I know this is by the book, and in reality the situation might be completely different - however the manual is a legality, so the ability of the crew should follow suit).
cheers,
starship
