There have been a lot of good suggestions for collecting shells when snorkelling, but no-one has mentioned (yet) the one I use.  Try an empty 35mm film container.  They are an excellent size for the smaller shells that are often found snorkelling (or diving), the lid comes of and goes on easily and the are rigid so delicate shells will not be crushed.  Another advantage is that they can easily be tucked inside swimming trunks or costumes so the lack of pockets on most such clothing is no problem.  Also, and best yet, they are free as photoshops just throw them away.
 
Tom Walker
Reading, England