If your child's doctor agrees, then whenever you are ready. I was an infant, 3 months, I think. My parents took me to the YMCA in Montreal in the 1950s. Supposedly, I learned to swim then and there. Not strokes, but dog paddle, kick and taking my face out of the water. My mum was with me the entire time, inches away. The one in my city suggests six months and the pool is 'therapeutic', warmer than a regular pool.
You need the proper diapers and diaper covers. You need to know the pool accepts infants and children in diapers. The pool and area needs to be clean. Personally, I would not allow anyone else to take my infant into a pool excepting the father and me. Doing under the direction of a swim instructor is a good answer.
If this is your own pool, then make sure it's clean and that the diapers are not soiled, are tight and covered. I would be careful if you have no experience, but simply holding a baby in clean water that is warm enough, should be fine.