Last time I was there a few years ago, 156.8 Marine Ch. 16 was very active. Lots of ship activity, port pilots, tugs, military, Coast Guard, etc.
BTW, If you get a chance, go out to Port Aransas and sit in the park along the ship channel. You'll be a couple hundred feet from the ships as they pass by and you'll likely see dolphins "escorting" the ships through the channel.