The horse mussel is called mexillon or choro here. The mussels grow in clusters of several to tens deeply embedded in the cracks of large rocks. Mexillones are abundant along rocky stretches of coast from the mean low water mark to fairly deep. The shallow water populations are decimated by local gatherers in many areas but plenty are available by snorkeling or best, scuba. 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.