We bring our own gear too. But if it comes down to space/weight do what Sonny suggests and just bring snorkel/mask. Rental isn't expensive but...yuck...
As far as sand fleas goes, I've never had a problem with them. We have been to St Thomas, St Marteen, Grand Cayman, Cozumel and Jamaica and Belize. Didn't do a beach thing in Jamaica or Belize but didn't have an issue on any of the others.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007