You can check this link why it is beneficial to learn how to swim at an early age: https://www.waterbabies.co.uk/baby-swimming/benefits-of-learning-early Bert Lozada has a program for babies as young as 6months: http://www.bertlozadaswimschool.com/programs/babies
I think it's really important for everyone to start learning how to swim as early as possible. It's not only a skill, but it can also have health benefits for the kids (most especially if they have asthma). I agree with Sam, Bert Lozada has an excellent program.
I don't think it's a necessity, but I think the sooner you get them acclimated with water the better! It'll encourage them to play more at the beach when you go! And they won't develop any irrational fears of water or anything along those lines.
Hmm, I think there's no problem about it if you really wanted to enroll your kid but like what Phil said, it's not really a necessity. Besides, there are a lot more things you can teach your kid at a young age beside swimming.