The ICW is patrolled by local law enforcement, Marine Police, Game Wardens, etc. State laws apply while in transit. Not a problem in gun friendly Southern States, potentially a big problem in states starting with Maryland, heading North.
Having a concealed handgun permit that is recognized by states you travel through is helpful, as is keeping a long gun onboard and unloaded.
Even an unloaded long gun might get you in trouble in some states, best to research thoroughly first.
I'd go with a pump shotgun, unloaded with shells handy on the buttstock.....