The resorts that line the South Padre Island beach put you RIGHT on the beach. We stayed at Saida Condos and spent most of our days on the beach. The beach is very shallow with sand bars every 20 feet or so out from the water's edge. Great fun for medium sized kids and adults that aren't strong swimmers because you can get the feeling of being "way out there" without ever going in over your head. When we arrived, the water was unbelievably warm, almost too warm at the edge. As the days went by the surf kicked up and the water temperature moderated, but only enough to make a dip in the Gulf of Mexico refreshing. The beach itself is immaculately maintained. There was no sign of litter or unpleasant sea-debris (seaweed, etc). Not too many gulls (probably due to the beach being so clean), just enough for atmosphere.
We did travel up the island to the undeveloped areas, but found it a little TOO wild for us and our two-year-old baby. Too much traffic on the beach (I'm talking cars and trucks here) and a lot of trash lying around. I usually go for less developed areas, but this was one case where it didn't pay off. There were more people there than in the resort areas.