There is no "arranged" swimming with dolphins anywhere in Belize and I truly hope it won't happen in the future. If you are so lucky as to encounter some dolphins while snorkeling, then it will be up to the dolphins whether or not you get to interact with them.
This is available in Roatan, Honduras at Anthony's Key Resort but I don't think the penned dolphins had any say in it.
Go snorkeling or diving and enjoy. If it happens, it will be spontaneous and fantastic.