re: What to use to bait shrimp, using a cast net.

Behind the weirs in the marsh. You don't just go out in open water and start throwing a net.

Although, I have heard of people going in the bay and sticking pvc pipes in the bottom about 20 yards or so apart and then baiting around the poles. Then just troll from pole to pole throwing a net. That way you can pin point where you had thrown bait.