I was wondering. A catfish is a fresh water fish but is being caught by saltwater fishermen. Is it legal to keep a catfish without a fresh water license even though you catch it in salt water?

Catfish are not solely freshwater fish. The freshwater variety is capable of living in brackish / saltwater conditions.

You don't need to run to the bait shop to purchase a freshwater license for accidently catching a catfish in other than freshwater, unless the area you are fishing has been designated a "freshwater" area.

The only area of concern here is whether or not you have the proper licensing for the particular area that you are fishing.