This was the thing I most looked forward to on our trip to Italy. Maybe for that reason it was a little disappointing. We did the snorkel tour of Baiae.
First, make sure you dress and prepare appropriately. We did. We took our own masks/snorkels, wore footwear that could be worn to the boat and pitched in a tub with everyone else's stuff. One American lady did not and she tried walking barefoot to avoid getting her shoes dirty/wet and ended up cutting the bottom of her foot on a piece of glass. They have masks, fins, snorkels, but we saw several petite people have problems with size. Also, I just wouldn't want to wear a mask/snorkel someone else has been breathing and spitting into, even if it is dumped into soapy water at the end.
Be prepared to wait. You will arrive a very long time before your tour starts. There will be chaos as the guides attempt to get everyone fitted with equipment. And people will arrive for up to about an hour or more past the time you think you are scheduled to go. So you will wait and wait and wait. And, then there will be sessions for the scuba divers before you can go. And then there will be a video about the site before you can go, snorklers and scuba divers go together. So, at first, it will be hard for snorklers to do much--the scuba people are right below. Eventually, the scuba folks move off and snorklers can get a better view. It is lovely being in the water and the mosaic is really cool. And our guide was good enough to pull up ancient Roman stuff so we could touch it.
But, I'll be honest, it was a long time at the Subaia office and a short time on the water. I'm glad I did it, but would probably opt for a different activity next time.
Also, you leave all your stuff in unlocked bins at the office while you are out on the water. Leave anything you don't absolutely need behind.