WDW has closed all of its beaches... I assume the Swan and Dolphin beaches are closed as well.
It is very possible and easy for a gator to get in this lake likely through a number of connected tunnel systems throughout the resort. To think a small body of water would not house a gator is not logical. Many, many small bodies of water near highways have gators that live in them. So it is quite possible this small body of water could have one.
I have small kids, and to think that these beaches are 100% safe at water's edge is just misleading. I think playing farther in on the beach is safer given the recent situation. It is because of representations like this, that the beaches were safe to play and wade in the water, is why this family suffered an unspeakable tragedy.
I think the appropriate thing for Starwood would be to take the photo down and shoot another photo not near the water's edge.