minnow seines in fresh and salt water....

So far as I can tell, you are allowed to use a seine net (small ones without the big pouch) for collecting minnows for either bait or aquariums (no game fish) or for harvesting shrimp.

I think all that’s needed is the appropriate fresh/salt license, but can anyone help me with this to be sure?

I mainly want to use a seine in small tidal creeks for shrimp, mud minnows, and whatever else I might catch (bait, and amusement for my young son).

the corresponding license will cover you if you are within recreational restrictions

Saltwater- didn’t see more on gear restrictions in a real quick search (http://www.dnr.sc.gov/regs/saltwaterregs.html):

SC gamefish (cobia, red drum, spotted seatrout, tarpon, and striped bass) may not be taken by nets. Sharks may not be taken by gill nets.

Nets may not be set more than halfway across any waterway at any time.

Stationary or fixed nets, including gill nets, may not be set or placed within 600 feet of a net previously set.

It is unlawful to use any seine or gill net in any waters within a state park; except, small hand seines and cast nets for taking shrimp are allowed.

Freshwater (http://www.dnr.sc.gov/regs/pdf/freshfishing.pdf):

Cast nets for catching nongame bait fish may be in possession while fishing for game fish.

Minnow seine: Must not be greater than 4 feet in width by 20 feet in length with a mesh size of not more than 1/4 inch square mesh. It is unlawful to have in possession a minnow seine from 10 PM to official sunrise.