For 21 yrs now, I have gone to Fiesta Tx in SA and Schlitterbahn in New Braunfels. In my opinion, Sea World is very expensive and it is all about the shows and kids your age want ACTIVITY! My kids are older but I still don't like Sea World.
The actual beach in the city of Corpus is somewhat easier to get to but seems a little dirtier(trash). But there is public beach area right next to the Aquarium in Corpus. The Aquarium is way cheaper than Sea World and more hands on. Then if you go to the T-head area, you can walk along the walk, see the fishing boats. Another fun thing to do is a boat that goes out on a tour of the bay-open air on the top and closed in with AC on the bottom. If you are really serious, I can look up the name for you. Corpus has a 40 things to do in Corpus on facebook.
If you travel 15 miles more, you can be at Mustang Island. They have public bath areas to rinse off before getting in your car to drive so you would not have to stay in a costly motel on the beach. But it is a State park and has entrance fees.
Just remember to buy your beach toys before you get there cause they are much higher! Make sure you bring a baggy for shell collecting! We would collect them and then come home and paint them!
I live between San Antonio and Corpus and love to play tourist in both towns. SA has a S.A.V.E. program which gives great discounts (from the Visitors center). Downtown has the river boats, the tower with interactive 4 D movie, no fee at the Alamo. Some of the other downtown tourist places have high fees and probably not good for kids your age. But they do have a super nice kids museum downtown that my kids always loved and spent an entire afternoon there. Very hands on. Then you have the Zoo with a train thru the park and the Witte Museum with a kids zone. And a great kids theater, Magik Theatre.
