My DD just got them in the autumn, shortly before her 3rd birthday. She had poor hearing due to glue ear but had not had ear infections.
It was extremely simple. The operation itself (both ears) took less than 20 minutes. The anaesthesia is scary but we stayed with her while it took effect and went in as soon as she was awake. She was very out-of-sorts (but we had been told to expect that) and then she had a short sleep on the ward and was her normal self again.
With regard to your questions, my brother got them in the 80s and he couldn't get his ears wet for a year or so -- could only go in water with earplugs and swim hat and couldn't go under at all. Our consultant told us this is outdated advice and we don't need to take any precautions with DD. I would do whatever the consultant says.
With regard to flying, the child will have no discomfort from pressure whatsoever -- the grommets take care of that! Our DD flew less than a month after her operation.
I think the operation probably leaves a bit of discomfort. We gave Paracetamol afterwards but she didn't appear to need it even the next day. The grommets don't cause pain once they are in. We had to keep her isolated from other children for a few days to reduce exposure to colds, etc.
I think any medical procedure carries risks, but the risk of years of painful ear infections must outweigh the risk of grommets or you wouldn't be offered them!
I can't give technical information about the surgery but there is a lot of information on the internet with diagrams of the inner ear and where the grommets get inserted. You might be able to watch the procedure on YouTube if you have the stomach for it!
We had not noticed any hearing issues with DD but there has been a massive improvement now. Her speech is better and she is constantly asking me what sounds are -- obviously she was missing out on most of the background noise in her world. I was hoping she would obey better but alas that has not happened. . .
Good luck!