There is no "Legal requirement" although I am not sure what you mean by this?
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.
In reality, it is one of the least difficult obstacles to overcome and there is a very good chance that learning to swim will not only enhance your enjoyment of life, but might actually save somebody else's one day.
In my opinion it is shameful that (in the U.K) so many schools fail to teach swimming as part of the curriculum these days, but that shouldn't stop anybody from learning to swim.
An airline doesn't have to employ you because something isn't a "legal requirement" but most will expect that you can swim at least a specific distance unaided. Rather than wasting time arguing the point here, I would suggest (as others have) that you sign up for lessons. Compared to the cost of most other aspects of this career it will be (ahem) a drop in the ocean!