Just did a similar cruise last month. If you are going to dive two ports, Bonaire and Curacao would be my top picks without hesitation. I skipped diving at Aruba to snorkel with my wife, so I cannot comment on diving in Aruba, but from what I saw snorkeling, diving would not have been great in Aruba. I did dive Bonaire and Curacao, and had great dives at both places. I am over 70, so was too old to do the ship's dive excursions at Curacao and Bonaire. Instead, I did a wonderful two tank shore dive with Dive Friends Bonaire, and a similar two tank shore dive with the Dive Bus in Curacao. I can highly recommend both of those dive operations. Dive Friends Bonaire is right across the street from the cruise terminal, and you might be able to talk the Dive Bus staff into fixing you up with transportation from/to the Renaissance, which is a short walk to the right of the Mega Cruise Ship Terminal. If not, you would have to grab a cab. Unless the ship's Bonaire dive op is taking you to Klein Bonaire, I do not see a big reason to do the ship excursion, other than not having to deal with with changes or cancellations should the ship not dock or is late. At Curacao, Oceans Encounters was the ship's dive op on several prior visits to Curacao, and they were excellent as well, so I would not hesitate to use them to avoid the taxi to/from the port in Curacao.