You need a tool like a strong diving knife to work the mussels loose from their perch in the crevasses. Horse mussels are dark fleshed and strong tasting. Thai snails or caracols are relatively easy to gather in the rocky intertidal zones. Just pick them up carry them back to camp or home and put them on a grate over a fire. The snails cook and simmer inside the shells and when done, they are easily removed with an ice pick, screwdriver or similar tool.
Snails are mostly intestine, except for the meaty foot.
Some people eat the whole thing. Caracols are available out of the shell in almost all supermarkets and seafood specialty markets. But the taste is lost and it is just not the same. Octopus is very common here and you can get them while snorkeling or diving. At night when the tide is low and if the water is calm you can get them in shallow water along rocky shores.