It takes about 15-20 minutes to tender into the city. The tenders run non-stop, but it's a bit of a wait in the morning trying to catch a tender. Everyone wants to get off first thing. If you wait for about an hour, things will have calmed down a bit. You would have maybe 1-1 1/2 hours to walk around, then you'd have to catch a tender back to the ship. I was going to do that after we got back from our snorkeling trip, but the weather got bad and they stopped the tenders for about 1 1/2 hours. By the time they started up again, I would have basically been able to ride over and turn around and come back, so I just stayed on board.
I guess that's another thing I should have mentioned. If the weather is nice, you'd have enough time to go ashore and return to the ship for your snorkeling trip. If there's a chance of a storm blowing up, you might not get back in time. Just a thought.