We went to the Sian Ka'an from the south entrance and there is only a beautiful beaches and Mangroves.
There aren't any lodging or restaurant in the south end of Sian Ka'an, but I had found a Hotel on the site www.mahahualhotel.com directly in the city of Mahahual.
The reserve is fantastic, crystalline waters and white beaches... it's proper for the people to which it likes the adventure. I have seen many birds and a lot of iguanas.