These photos were taken in the water around San Salvador during a dive trip April 18-25, 2009
It looks to me like the egg case of some sort of Whelk.
See this other picture i found
http://images.google.com/imgres?imgurl=http://donchesnut.com/travels/bahamas/bah300c.jpg&imgrefurl=http://donchesnut.com/travels/bahamas/bahamas.html&usg=__vzOKBMP7DohKHumYKYzDUQNV4Uw=&h=285&w=364&sz=29&hl=en&start=17&um=1&tbnid=b8aeo1R8T0TrXM:&tbnh=95&tbnw=121&prev=/images%3Fq%3DWhelk%2Begg%2Bcases%26ndsp%3D18%26hl%3Den%26safe%3Doff%26sa%3DN%26um%3D1
It looks to me like the egg
It looks to me like the egg case of some sort of Whelk.
See this other picture i found
http://images.google.com/imgres?imgurl=http://donchesnut.com/travels/bahamas/bah300c.jpg&imgrefurl=http://donchesnut.com/travels/bahamas/bahamas.html&usg=__vzOKBMP7DohKHumYKYzDUQNV4Uw=&h=285&w=364&sz=29&hl=en&start=17&um=1&tbnid=b8aeo1R8T0TrXM:&tbnh=95&tbnw=121&prev=/images%3Fq%3DWhelk%2Begg%2Bcases%26ndsp%3D18%26hl%3Den%26safe%3Doff%26sa%3DN%26um%3D1