sassychic, it's perfectly safe and it's extremely beautiful. Our family has been going to Chankanaab National Park for 20 years. Our daughters were toddlers when they went for the first time.
I can't say enough great things about it. We always snorkel there. You can grab a cab at the pier. The taxi's run back and forth all day, so you shouldn't have any problems finding one.
We're so used to Cozumel that we always rent a car at the pier and go it ourselves to the Park. It's approx. 15 minutes from the ship. Nice thing is....it's a straight shot down the road so it's impossible to get lost getting there. You can follow the signs posted for the Park.
Keep in mind, they will close the Park off if there are high winds on that day. We've experienced that once before.
You will find food and beverage, showers, lockers, souvenirs, nature paths, lounge chairs (bring your towels from the ship), beach wait staff. It's pristine and I would highly recommend Chankanaab for the day.
Have a great cruise.