My one-time experience suggest it is boat diving.
Also, I don't regard it as one of the beter dive-destinations
I've visited.

Here isn the Ilhabela section from my November, 2000, Brazil trip report:

       ...
         Ilhabela

         Saturday afternoon we drove to Ilhabela, an island resort
         roughly east of Sao Paulo. I'm not sure what I had expected,
         but what I found, once out of Sao Paulo was beautiful mountain
         scenery. The last few miles leading down to the coast were noth-
         ing short of spectacular, even though it was cloudy and we had
         intermittent rain. In this part of Brazil, the mountains come
         right down to the ocean. There are only minimal planes, mostly
         between steep headlands that extend right into the ocean.

         After reaching the coast at Caraguatatuba, we continued south to
         Sao Sebastiao, where we took the ferry to Ilhabela. Found our
         "bed and breakfast" -Pousada Colonial Ilhabela- with no problems.
         ("Pousada" is a term for a "Bed and Breakfast" hotel.) We got
         ourselves registered for SCUBA diving the next day. Then we
         went into town and had some Rum and Coke with Pao de Quays -
         a delicious cheese bread snack that is popular in Brazil. We
         returned to the Colonial for an early supper and hit the bed.
         I hadn't slept well Thursday evening...
         I got one of the best night sleep I had had in quite a while.

         We woke the next morning and enjoyed the Cafe da Manha (break-
         fast) before boarding the dive boat, the Neptune, at 9:00 AM.

         The Neptune is an old, slow wooden vessel, about 70ft in length.
         Most of the main deck was open and covered-plenty of room for
         the 15+ divers on board-It could have carried twice that number
         without being really overcrowded. Most of the divers were part
         of an open water class -A bit scary given the low visibility we
         experienced.

         Now here is a piece of advice-figure what weight you will need
         on your belt in kilograms BEFORE you arrive in Brazil. I had
         a new wet suit, virtually NO command of Portuguese, and the
         problem of converting KG to LBS. The end result of this was
         that I was WAY overweighted on the first dive-I carried 14 KG,
         which I later calculated, was about 31 LBS. WAY too much. It
         was because of this, I assert, that my wife and I got separated
         as we descended. This was also partly due to poor visibility -
         maybe around 5-10 feet. And, O.K., we were a bit careless.

         The dive was on the wreck of the Velasquez which had gone down
         in 1908. The wreck was pretty much broken up-with only one
         or two minor exceptions, all we saw were bits and pieces-no
         outline of a ship. Water temperature was around 72 F (22 C), a
         bit colder than we had expected, but O.K. for the 3/2mm suits
         plus vests we were wearing. But being overweight etc. We went
         through air more rapidly than we would have liked. Ended up with
         a dive of about 30 minutes to a maximum depth of about 43 feet.

         For the second dive I removed 4 KG, dropping my weight to about
         22 LBS. We dove another wreck, the Therezina, which had sunk in
         1920. Visibility and water temperature were about the same. (Air
         temperature was in the mid-upper 70s F.) We hit a maximum depth
         of 43 feet again for about the same time as the first dive. At
         that point, we were a bit board and maybe a little chilly, so we
         got back on the boat.

         The boat had a snack bar that served fresh fruit (plentiful in
         Brazil) and beverages along with pizza and hot dogs. Since we
         didn't get back till mid afternoon, this was appreciated.
                                                     
         In the evening we went into town and had a lovely dinner at a
         waterfront restaurant with a view of sunset over the mainland.
         Magnificent! Our meals in Brazil were all very good and inex-
         pensive. Even in Rio de Janiero, the two of us had dinner and
         drinks on the waterfront for only about US$25-30. Compare that
         to Miami Beach!!!
       ...

P.S.  I finaly got the weight right (low enough) the next day in Parati.

I've omitted that section, ask if you want the entire report.
Here is my "summary""

         DIVING SUMMARY-Brazil is a big country, similar in size to the
         USA. Obviously, our experience was not complete; it may not have
         been fairly representative. With that caution, I would say this:
         Don't plan a dive trip to this part of Brazil. There are better
         options. But if you are here, it is worth the effort to schedule
         a day or two for diving.