Your resort should have a lot of choice of tours. Snorkerling is one they must have. You can join the tour your resort offers, it is more expensive,but the quality is pretty good, most likely better than you go for your own.
And, I know a lot of resorts offer free transportaion service to major tourist attactions, Cozumel shoud be in the list. You can take the bus to Cozumel, and should be able to find a a lot of snorkerling tours, they are quite cheap, but the quality is so so.
If you take resort tour, they have their own transportaion, you can plan for half day.
If you go by yourself, you need a whole day.
I would personally recommend resort tour, the quality, safety is guaranted. And you save time, and other troubles.
If you like adventure and surprise, it is another story.
