I've enjoyed excellent shoreline shell collecting on several trips to Isla
Holbox, on Mexico's Yucatan peninsula.  If you're planning a shelling
vacation, you might enjoy our trip report:

http://home.earthlink.net/~jcaldeira/holbox.html

Unique ocean currents and minimal shelling pressure at Holbox allow a
collector to find nice specimen shells while hardly getting one's feet wet.
Good food & cheap beer in town, too!

-John

John Caldeira
Dallas, Texas, USA