I have been to the riveria maya several times, and have never encountered anything that I would consider a huge threat, not to say it couldn´t happen. I have not personally stayed at the Xcaret, so i can´t comment on that resort. All of the resorts down there are in the jungle, and no, they are not fenced off or anything like that. Now, what you will likely encounter are a lot of cats, iguanas, and various insects. Just pay attention to your surroundings, and you´ll be fine. Have a great time!