In some areas you must have a captain license to guide. There are even certain sections of river up in Michigan where once are a certain distance from Lake Michigan it is required.
Bob is indeed correct on correspondance issues. There is also an inland license as some lakes require a captains license. If your taking out clients be sure for above the demarcation lines. Check your lake rules. From a liability issue alone it is prudent. There was an incident near Charlotte with a guide having no license. An accident on the water did not go well for him so you never know. There is some gray area on inland waters.
My friend I fish with in Tampa is a guide, but does not run a guide business (yet), and he was telling me some of the things they have to know and do, as well as the time on the water you must document.