The recommendations which were made to me when I was looking into this outside a ship's tour are:
<http://westwater-adventures-barbados.com/turtletour.html>
<http://www.eltigrecruises.com/>
<http://www.funbarbados.com/Tours/supercats.CFM>
The recommendations were made by a gentleman who lives in Barbados for part of the year. Yes, these are snorkel trips and the costs are listed on the web sites.
Hope that helps!