I'll add that it depends what kind of lifestyle you're looking for. If you enjoy what many of us have in our minds as a stereotypical ski instructor lifestyle--living in a community where everything is about skiing and snow and camaraderie with your fellow ski instructors, not to mention those ski bunnies and apres ski parties--then the closest analog in the dive instructor world is probably the Florida Keys or the West Palm area. It's all about living on the water there. The dive industry is a major player. A place like Key Largo is small enough to provide a sense of community yet big enough to avoid cabin fever. Sure, there would be much competition with others seeking jobs just like you, but the flip side of that is that you can be a part of a vibrant dive community.
The Carolinas can be charming, the cost of living is low, but the pace of life can be a little slow there. Moreover, the dive industry is a relatively small part of the coastal life. For these reasons, if the Carolinas still appeal to you, then Wilmington, which you mentioned, might be a better choice than, say, an even sleepier place like Morehead City. No offense to DiverDownD3, but I think I would get cabin fever living in a small town like that. Another consideration is that the people who dive in the Carolinas tend to be more dedicated to the sport--more serious-minded--than the vacationers who flock to the Keys and South Florida. It takes an hour or more on a boat just to reach some of the sites. The weather can be unpredictable. Depending on what you're looking for, the serious-mindedness of it all may be a plus or a minus.
San Diego and Southern California in general are fun, but expensive. It's also a big place, of which the dive industry is just a tiny bit--there is so much going on there and so many things to do. The water is not sub-tropical like Florida. I learned to dive in San Diego when I lived there, but the chilly water turned me off from doing much local diving. When I think of San Diego, diving is not the first thing that comes to my mind.
Any reason you specifically want to stay in the US? The Caribbean would offer more options.
Of course, all of this is based on you first earning an instructor credential, which you could do in the first summer, but just that and nothing more--no experience, no special skills like marine diesel mechanic--is going to make landing a job a real challenge. It may be that you spend this summer becoming an instructor, and you don't land a paid position until the following summer. Just trying to be realistic.
