Hi Mary Jo,
    I just got back from a trip to Nassau and did a lot of snorkeling and
shell collecting around Nassau and Paradise Island.  It was my first visit
to the Bahamas so I can't speak for the other Islands.  Sadly, the
reputation for not so good shelling in Nassau/Paradise Island was backed up
by my experience (although I didn't get to the south side of the island
where I imagine there is less polution).  There is a cove near Atlantis
(Love's beach) where I found  some good shells snorkling and on the beach.
Also you can try a beach further East on Paradise Island called Snorkeler's
Cove.  But all in all it was pretty slim pickings.  I would recomend just
about any other Island to visit.  If it had to be Nassau though, I would
recomend visiting nearby Blue Lagoon (Salt Cay) Is. (Ferry from Paradise
Island about a half hour and $25).  That is pretty much where I found all
my good shells in some quantity.  There is a little human made secluded
harbor area on the south side of the Island where I snorkeled in shallow
water and found many excellent shells (including 3 Cassis tuberosa's - one
amazingly over ten inches! - he was a bit worn and I didn't know how I
would go about transporting/killing him so I decided to only take his
picture).  I also encountered a huge Manta (Sting?) Ray kicking up a bunch
of "smoke" flapping his wings on the ground.
-Take care, Kevin