Look at the beach in narrow bands
A Gulf beach can seem empty until you slow down. Different parts of the shore hold different clues, and the line between them moves with wind, waves and water level.
At the high wrack line, look around deposited seagrass, driftwood and other natural material before stepping through it. The National Park Service's Padre Island beachcombing guide explains how a wrack line forms and why it gathers organic material. That source covers Padre Island National Seashore, not the City of South Padre Island, but the habitat clue is useful across the region. Leave occupied shells and living animals where you find them.
On the drier sand, small holes and tracks can be signs of Atlantic ghost crabs. Ghost crabs spend much of the day in burrows and are more active after the beach cools. A quiet look near dusk may reveal movement. Do not pour anything into a burrow or chase an animal for a photograph.
Where waves wash in and drain away, the sand is continually being sorted. The mole crab and beach hopper guide explains the small burrowers associated with the swash and damp wrack. Watch the sand surface and receding water instead of digging up a patch to see what is underneath.
Rocks and lagoon shallows are different habitats
Jetties add hard surface to an otherwise sandy coast. Barnacles and jetty life introduces the animals fixed to or sheltering around rock. Observe from a stable public surface; wet, algae-covered stone and passing swell can make the outer rocks a poor place for close inspection.
On the Laguna Madre side, the water is not simply calmer Gulf water. Texas Parks and Wildlife describes a hypersaline lagoon where salt-tolerant seagrasses and algae support crabs, shrimp and fish. Start with blue crabs, swimming and box crabs and Laguna Madre seagrasses if you are visiting a public bayside boardwalk or launch. The Laguna Madre bayside guide separates documented access from shoreline that only looks reachable on a map.
Not sure what you found?
Photograph the animal in place from above and from the side if you can do so without handling it. Include a nearby natural object for scale, and note whether it was on dry sand, in the swash, on rock or beside the lagoon. Shape, movement and habitat often help more than color alone.
Season, water movement, temperature, recent surf and time of day all change what is visible. If the find is an empty-looking shell, start with hermit crabs and watch for movement before assuming nobody lives inside. Leave the animal where you found it and use the habitat notes in the field guides to narrow down the identification.