Shorebirds, much like seabirds, is more of a functional classification than a taxonomic one. Shorebirds are those that live in, on, and around the shorelines and beaches of the world, and include birds like waders, dippers, herons, and plovers amongst many others. Many of these birds are water birds (i.e., they can inhabit freshwater, brackish, and marine habitats) rather than specifically requiring marine areas only.
You can tell a lot about the likely prey that different birds feed on, and how they get it, by looking at their beak shapes and lengths. Birds like oystercatchers and curlews have long bills used for probing deep into soft sediments to fish out worms and other invertebrates for example. Those with shorter bills might feed shallower, or on near-surface and/or on rocky shore invertebrates like amphipods and the like.