There are tons of tours that are available to swim with the sea turtles (and other marine animals) in this area. Our hotel actually offered one that cost slightly less ($5-$10) per person, for example. Having said that, PICK THIS TOUR.
We booked this excursion somewhat at random out of the available tours and ended up with the best possible experience we could have ever asked for. It was a private group tour that consisted of just us for the day, which was spectacular. No waiting on anybody else to swim along, and nobody to make you feel rushed.
Our guide, David, texted us the morning of the tour to ensure we would be there and would know where to go. Unfortunately, I missed that text, so they followed up with a phone call shortly before the meeting time to make sure we could find each other--he was generous enough to come all the way to us to meet and get ready. Once we got in the water, he made sure to not leave me behind (as I'm not the strongest swimmer) and did an incredible job scouting and pointing out the turtles, rays, and other items (Spanish Cannons!) that were there. There were a couple of times that the larger group tours swimming around us missed seeing turtles within a few feet of them that David pointed out for us.
The experience itself was pretty unbelievable as well--one that can't be replicated in many places. All in all, both the activity and this particular tour are not to be missed. If you're waffling on whether to go--just book it and enjoy.
I will say that this tour does not include any time on a boat, so you're both swimming out to the main tour area and swimming back in, so make sure you're able to swim/float (with a life jacket) a decent amount of time. The good news is that you're generally so engrossed with what's around you that you don't notice too much.
I can't recommend this tour and David enough--we'll definitely do this again the next time we're in the area. Thanks for the fantaastic experience!