I got curious on the whole subject and kept reading so here is some more info. It is a violation to take more than 15 live univalve snails during a day which include no more than 2 each of the following snails: lightning whelk, horse conch, Florida fighting conch, pear whelk, banded tulip, or Florida rocksnail.
I think you can actually get in trouble for transporting water from the coast because it could contain hitchhikers. Also there is a permit process to collect sediment materials. It appears as part of an erosion control plan. I'm not sure how you can take sediment without hitchhikers either but that is the way I'm reading it. These rules about water and sediment I have only found on the Texas Administrative Code so it is kind of hard to understand. I'm sure the Texas Parks and Wildlife could answer more questions or at least direct you to the right place to get them answered.
There is just a lot to know about going down and taking stuff from the coast that could get you into trouble if your not aware of the laws. Just FYI.