I'll offer another opinion. Beach directly in front of the Marriott is terrible. Very shallow (from hotel to water). Also, tons of ironshore if you go in the water directly in front of the hotel -- that you will stub your feet on, run into if frolicking in the surf , etc. If you look closely at the Google maps images, you will see it. Or see it here:
http://www.panoramio.com/photo/31426048
Hotel itself is great, and I understand recently renovated, very nicely. But they can't really change that beach.
There is good snorkeling, if you get in the water past all the ironshore, and then snorkel over to the left. Two reefs , out about 100-150 years, between Treasure Island and the south side of the Marriott. Also visible from Google maps satellite view.
If you do go there, I strongly recommend investing in water socks/ aqua socks for everyone who is going to spend significant time in the ocean. You'll see why when you get there. Buy them here -- lot cheaper than the price you will have to pay for them in GC. Something like these
http://www.amazon.com/Speedo-Womens-...ds=water+socks
The beach pales in comparison to that of the Westin further north on Seven Mile Beach. IMO.
That being said, GC is a great island -- very safe to walk around with your kids in the evening and visit lots of other restaurants. Nearby the Marriott is Deckers, which I highly recommend.
We've been going to the island for about 25 years, and know it pretty well at this point.