Most of the excursions include the equipment in the price if is a snorkel specific tour. Some tours offer with/without snorkel. And some places do rent it out.
The quality is pretty good for the snorkel tours. And is better than Wal*Mart. I do bring my own when not included in my tour, and when included I still bring my own mask and snorkel. I don't trust their sanitization..no germ jokes please from the peanut gallery
U.S. Divers is the minumum brand to get in my opinion. I did get a better quality mask, and I bought a vest last year. If you are going to cruise at least once a year I would say get your own.