If it's not a legal requirement then the airlines cannot force anyone to do it
Well we operate under UK rules and FWIW our Ops manual (i.e. legal requirement) states that for Initial Training the trainee "swim with lifejacket", which sounds simple enough but the same legal document also states a requirement for the trainee to demonstrate "Donning of a life jacket ....in water", which is going to be pretty difficult if you can't tread water/swim.
On a practical point regardless of the law I agree with the point made by Beazlebub that:
An airline expects you to be an asset in an emergency not a liability. In other words it shouldn't in those circumstances fall to your colleagues or other passengers to rescue you because you have failed to learn a basic life skill.
...especially as a one day you might be Pilot in command in which case do you think some teenage cabin crew member or a pilot junior to you on the senority list is going to rush to rescue you..
I would gently suggest that rather than trying to argue a point of law either here or elsewhere you need to accept that for any crewmember some degree of confidence in and around the water is a must. It may be something the Safety Equipment and Procedures (SEP) trainersare looking for, and that time/money spent on swimming lessons might be a good thing....TBF you don't need to be an olympic swimmer, just learn to swim a length...