We bring our own mask and snorkel. Every excursion we've been on offers all as part of the tour. We don't feel comfortable about strangers cleaning snorkels & masks so we bring our own and use their fins.
Renee - The better half of RobRen :-)
Carnival - Spirit: Alaska (August 8, 2007)
Carnival - Miracle: Exotic Western Caribbean (April 9, 2006)
Royal Caribbean - Explorer of the Seas: Eastern Caribbean (January 2005)
You really do need fins if you are going to cover much territory. Fins are also safer, in that you tire less easily and can get back to shore w/o a problem.
Lugging them along, or renting them from a vendor at the beach, depends on the room in your suitcase. I've done both. Also, if you are booking excursions, your gear is included in the price. If I plan on using fins three times on a cruise, I would probably bring them along with me. Not necessarily because of rental fee, but because my snorkel fins fit me better, and I don't have to spend my snorkel/beach time looking for a rental counter.