I've been to Schlitterbahn at South Padre Island once. I enjoyed it! If you have ever been to the original park in New Braunfels, Texas, you will be very familiar with the fact that the original water park is comprised of many tube chutes. Schlitterbahn at the beach is also this way, but looks a lot more modern.
It is located very close to the shore; one can get access to the beach at the park. When you first walk in, there is a wave pool where plenty of empty inner tubes can be found. Most, if not all, of the rides require having a tube, and there is a choice of one-person or two-person tubes. A river runs around the entire park, and you can have access to all but one ride in the park without ever having to get out of the river. The river in itself is fun because there are parts of it that have "rapids." There are not any horribly extreme rides that involve slides that are hundreds of feet high, but I found most of the rides to be very enjoyable.
This place is mainly geared towards families. There are separate areas of the park that are geared for younger children who are too small to go on the tube chutes. Children's tubes (smaller with netted bottoms) and lifejackets are available. There are lockers, changing facilities, clean restrooms, and tables available for picnics. You are allowed to bring in your own food, but there are also a few places where food can be purchased. I recommend bringing food in a cooler because the food within the park is somewhat expensive. There are also plenty of lounge chairs for those who want to take a break from the water and catch some sun.
Admission is roughly $30 for adults and $25.50 for children ages 3 to 11. If you are from Texas and frequent either Schlitterbahn New Braunfels or Schlitterbahn South Padre, a season pass that is valid at both parks is available for $113.00 (adults) or $75.00 (children).
For more information, go to the park's homepage:
http://www.schlitterbahn.com/intro1-spi.asp
I highly recommend going to Schlitterbahn at least once during your visit to South Padre Island.