Based on our experience with the glass bottom boat in Nassu, I would skip it and go for the snorkeling.
However, as far as snorkeling on your own, the only place I know you can rent the equipment is on
Castaway Cay. So if you want to snorkel anywhere but Castaway Cay, you'll have to either do the tours or purchase your own equipment.
As for equipment, everything is supplied on rentals and excursions, but here's some things to consider.
1. Do you want to put a snorkel in your mouth that a hundred other people have had in their mouth (even if it has been cleaned)? If you try to purchase and take your own snorkel, only your mouth will have been on it, and you can get snorkels designed for each member of you family. As an example, when my wife and I went to buy snorkels, she had never snorkeled and didn't regularly swim with her head below water. We purchased a "DRY" snorkel for her (if the tip goes under water for a moment, a valve closes so water can not run down the tube). Additionally, purchasing snorkels ahead of time will allow the family to get used to using them in a swimming pool before going out in the Ocean. Good snorkels will run you about $20-40 each.
2. Masks can be difficult to get fitting just right without unreasonable leakage. Different masks will fit different people in different ways. If you buy your own masks from a local dive site, you'll get professional help in getting a mask to fit your face that will be comfortable and will not leak (and you'll have it sized for your face before you leave home). However, decent masks will run you about $30-50 each.
3. Like masks, finding fins that fit your feet just right can be difficult. Its nice to not have to dig through the fins supplied on board to find a pair that fit. However, a decent pair of fins will run about $40-70 a pair.
4. To have a complete set of snorkel gear, you have to have the snorkel vest. It's sort of like a light-weigh inflatable life vest. Its purpose is to allow you to inflate it so that if you have something to help float if you start to get in trouble (swallow some water) or get tired. Disney requires them when snorkelling in their snorkel lagoon on Castaway Cay. A vest will run you about $50-75 each.
It is possible to find some of this gear cheaper if rather than going to a Dive store you try a sporting good store. Then you can get masks and snorkels for about $10-$25 each and fins about $20-30 each pair. But the quality will not be as good (fins might be easier to fall of, masks more likely to leak), and it will most likely not have the opportunity to try out different masks to see what fits your face the best.
What we did (of course there were only 2 of us) was to purchase quality masks fitted to our face and quality snorkels for our first trip. The next time we when on a trip, we purchased the fins. Finally, before our third trip, we spent the money on vest. Now we have our own set of snorkel gear (at a total cost of about $300 to $350 per person).
Perhaps what you might want to try is getting the cheaper mask and snorkel from a sporting goods store for the kids (they eventually out grow it and need more) and perhaps spend a little extra for the adults. Then you'll have the basics to practice snorkelling in a local swimming pool. You can then use just the fins and vests from the snorkel tour boat this year. If you got back in the future, then start looking into the other pieces of gear.
