You can bring your own gear. There is some debate on whether you can bring just a mask and rent fins or if you have to rent all or none. My own experience was that you had to rent the whole set, but others say they had a different experience.
As for the snorkeling area, I'd say it depends on your expectations and experiences. Disney has a nice area set aside for snorkeling. It starts at the beach and goes out fairly far in a horseshoe configuration. There are several lifeguard towers out in the water spaced out, and there are a number of yellow and orange marker buoys to mark things that Disney has sunken there. They are fun. Yes, when we were there, there were lots of fish too, but not like TONS of super rare fish. So it depends on your expectations. My teenage boys and I found it fun. It took us about an hour.
The vests that you have to wear (or bring your own) are useful if you aren't familiar with such things. Not only are they bright yellow to make you easy to spot, but they have a manual inflation tube so you can have it inflated a bit to keep you horizontal as you swim so you can look down, and then it can be deflated easily so you can dive down look at things, and then blow it back up when you resurface.