I've just returned from a "Girls Only" weekend in Corpus. I have a friend who has a condo right on the beach on the island. While this particular complex does not rent out, she often reminisces about staying at Island House when her kiddos were small and her parents were in the condo. Island House is right on the beach on the island. It has both a baby pool and a larger pool. While, I've personally never stayed there, my friend who has, just loved it (and she is a native of Corpus). There were also other hotels on the island -- a Holiday Inn, which was right on the beach and next door to our complex. You do want to stay on the island in Corpus and you will need a rental to get from the airport to there. I'm not sure where the marina is; but, I do know there is a "beach" in Corpus city proper. My friend says that's definitely where not to stay. If you do go to Corpus and stay on the island, there are several really good places to eat that are so very casual -- Snoopy's, Doc's, Italian House, there is a local pizza place on the island that will deliver, as well.
Have a great trip!