Hi 100p,
Although my extensive diving background was acquired back in Canada with some other dives in Hawaii, California, Oregon, Washington and Florida as well I have not done any diving in Brazil since coming here ten years ago. I still hold a PADI Openwater Diver Certificate and took several courses in Canada covering night diving, cave diving, wreck diving and ice diving. Unfortunately due to a visual disability I was forced to give up the sport I loved so much after logging over 300 hours of dive time under various conditions.
I have heard that diving here in many locations is spectacular and Brazil offers opportunities for both saltwater and freshwater dives that are extremely interesting. I have heard a lot about places like Ilha Bela, Ubatuba and Fernando de Noronha for incredible saltwater dives. Here in São Paulo there are a number of quarries and reservoirs that are used for freshwater dives. I don't know much about other locations.
I would agree with you that São Paulo would be one of the best places for you to learn Portuguese (speaking as a language teacher) firstly because of the abundance of teachers available. Also São Paulo lacks the regional accents that you will find in many other states. One learns a purer form of the language here that would be better understood anywhere else. If you start mixing a "gringo" accent with some regional Brazilian Portuguese accent you could end up with lots of troubles being understood when speaking.
You can avoid the hectic pace of São Paulo by living in one of the neighborhoods more removed from the city center, also you will cut down on the cost of rent at the same time. If you don't need to go into the heart of the city for some specific reason you will find that you can get almost anything you want or do whatever you want to do without having to leave your neighborhood, including studying the language. So don't worry too much about the fast pace and chaos.
I don't know about the diving scene in Recife, but if you look at any reports it doesn't take too long to find out that it is Brazil's highest ranking location for shark attacks. If you plan to dive there I would recommend that you find out everything you can from the locals, from divers... not just from dive shops or tour operators who have a vested interest in filling tour vacancies. Get the straight goods from those who frequently dive there. Hopefully you will soon be getting other replies to this thread that will help you out.
Cheers,
William James Woodward - Brazil Animator, Expat-blog