If you are only going on vacation, your current license is probably all you need. If you move and are legally classifed a resident of that state (these definitions can vary from state to state) then you need to obtain a resident boating license.

I'm not aware of any licensing requirements to operate most small personally owned watercraft. Most states have age requirements that apply to various boat lengths and engine horsepower, but other than that, generally the only other requirements that I know of imply that there be an adult present (or operating) boats that exceed the limits. And that the adult not be drunk. With that said, I would certainly and always recommend that any operator be fully aware of the rules of the road, aware of the safety features of his craft, knowledgeable about seamanship (and especially aware of the local waters and hazards) and keep an eye on the weather -- knowing what to look for and how to judge conditions. In short, take a boating safety class, and google "watercraft operator licensing" for your state or locality. If you've done that and feel out of practice, then take it again. If we're going to maintain the laissez faire attitude of state governments to small boating (which I certainly welcome), then the only way to do that is for each operator to be personally responsible -- and feel that way -- each time he goes on the water. And look out for the other guy, too.