Between Ft L and Boca there are tons of properties for sale, undervalued because of the depressed housing market. The diving is great, and even a little better, IMHO, slightly to the north in Boynton to Jupiter area. You can get great housing prices in Boynton. Yes, the further from the ocean (generally) the better the deal. Also, it seems like the houses get bigger and bigger as you move further away. Far away from the ocean is 10 miles away from it, not really very far. To get an idea where YOU would not want to live, you should probably drive around or confide in a real estate agent what wouldn't work for you. Good luck! I moved to S FL in 1999 and haven't looked back!
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My experience is pretty limited, but anywhere from Ft. Lauderdale on the south end to Jupiter on the north end might work. I am not an instructor, but only a lowly regular diver, so I am not sure what your considerations are. There are many shops and dive boats to get affiliated with all along the coast, if you hope to instruct. I live in and love Lake Worth, Florida. Why? Cuz it is funky, cute, diverse, has weird politics and is close to all the best diving. I have lived in Boca Raton, Palm beach, and West Palm, too. Spend some time checking out the various communities and see what you like. Check out Lake Worth, but only if you are a little nutty.
Good luck!