I didnt see your question at all before. But I most definitely have answers for you....
Yup! I've heard of it. Where I am, we teach it buttttt we begin with a few other things first. Well I don't teach it because my certifications ran out while I was pregnant last year but my sister still teaches it. She starts out with teaching Parent/Toddler swim lessons, even though there are children in the class as young as 6 months. She doesnt do it before six months. You have to teach the baby to hold their breath when they go underwater though or you will have a problem on your hand. How we usually start that at 6 months is you are in the water with your baby, face to face and they have a grip on both index fingers of both hands. Next, you blow in their face and let go. When you blow in their face they automatically shut their eyes real quick and hold their breath. Its a natural reaction. Then you let go and they bob under the water and come back up.
My suggestion, is that if you dont think that you can handle that (my mom said she tried to do it with my sister and I but it made her too nervous. I have had moms take their child right out of our program due to that making them too nervous) then you should not do the lessons. I'm not trying to be harsh but sometimes it does make the parent too nervous, seeing this. But we always require that the parent be present during the lesson and participate as well. It might be a little different with the ISR on that part though.
However, if you think you can do all that...I reccommend it with everything I've got! It is very important to me that my son learn how to swim and I plan on starting it as soon as its warm enough to be in the water (I wanted to start at 6 months, but don't have a membership to the Y and I live in NY so we obviously go through winter). I think you should do it. Its great that your infant know how to swim, with anything that can happen near a pool in the summer or even in a bathtub! I mean, obviously you will still need to watch your child near a pool in the summertime but it will ease your mind to know if he does fall in, he knows what to do and how to swim/keep afloat.
ETA: I didn't mean to come accross as rude if I did when I asked if you would be able to handle it. I just know that I've seen mothers who have had no problem and then other mothers who actually would get mad at me. Not saying you would get mad but to be all honest, I dont even know if I would be able to do it yet. From the sounds of it, if the baby did that good over the summer I would DEFINITELY go with the ISR.