I'm not sure, but on their site theres a picture of someone in a Scuba suit in the pool, so it could be some sort of attraction.
It's cool but I would prefer a fresh water pool. Swimming in salt water isn't much fun unless there is surf or you are diving. I hate the sticky feeling after getting out of salt water.
There is one is a small town out in west Texas that is pretty large itself. Not near as large as this one but it has been around since the 50's. You can scuba dive in it...... http://www.tpwd.state.tx.us/publications/parkguide/rgn_bb_004.phtml
I couldn't tell, but does it have any cross-overs? If you wanted to actually go to the beach it would be a hike otherwise.