Metal detectors are prohibited on PINS. I think it mentions it on the official website somewhere. I'm pretty sure you need a permit from the state for crabbing (with traps). Cast nets and surf fishing are legal with a fishing license and a saltwater stamp. The only crabs I've seen there are little ghost crabs that my kids love to catch and release, but they're too small for dinner. You can get all the TX fishing laws at the TPWD website here:
http://www.tpwd.state.tx.us/
A couple more things: It hasn't been specifically mentioned, but bring some sort of sun canopy to pitch for shade. There are no trees or other shade sources on the island. Insects aren't very bad, because the constant breeze keeps them at bay. If the wind dies down, you'll need bug dope.
Also bring plenty of dry clothes. Nothing you hang up will stay dry, and anything dry you hang up will soon be impregnated with moist sand. You mentioned bringing a cast-iron pot. If you don't keep it well-oiled the entire time you're there, you'll have a very rusty pot very quickly. I'd go w/ stainless or aluminum.
Everything you bring (including your dinner) will get sand in it, no matter how careful you are. Have a sense of humor about it, or it'll ruin your trip.
Bring more water than you thought you'd need.
There are a lot of medium-sized sharks in the surf areas where there are a lot of fish. The water is murky, too. Never heard of a shark attack on a human there, but be aware of it--especially if you have kids along.
Have fun. As soon as you get back, you'll probably start planning your next trip.
Greg