The 3 month thing is to separate kids who just have a bad cold and temporary glue ear from those who actually gave a problem with it.
DS1 had watchful waiting from about 2. His hearing wasn't too bad and he wasn't being too affected by ear infections. The consultant one day just said "I'll list him for grommets" with no consultation at all, he must have been about 5? I actually refused as I felt he wasn't being affected much by it, and he had had several other operations and I didn't want to add one when I felt it wasn't necessary. We went back six months later and his hearing was fine so actually he didn't need the grommets and I made the right call.
DS2 had grommets fitted in yr1. He had had worse hearing loss but again not too many infections. He had also needed speech therapy when he was 3 for about a year. The grommets were great, he woke up straight from the operation and said "wow, I can hear super good". The difference was amazing.
With regard to swimming, we were told no swimming for 6 weeks then go for it. All was fine for about a year until we went on holiday and he got a really bad ear infection from a hotel pool, which threatened us being able to fly home. It was really yucky with pus oozing out etc.
After that bad infection we found that he couldn't really go swimming without getting an ear infection. We tried ear plugs etc but nothing seemed to stay in properly and we couldn't find a good solution.
The following year we went on a holiday in a lifetime and the consultant gave us precautionary antibiotics to take with us so that DS2 could swim. I asked what the risk was to DS2 and he said there was no risk to the ears just the pain with an infection.
So anyway they were quick and easy and for a year or so he could swim fine. After a bad infection though he didn't swim regularly for about 2 years as the grommets took 3yrs to come out. They were the right thing to do despite that though.
Good luck.