Don't be hesitant about using the ship's dive op for your dive and snorkel excursions. I cruise a lot, have used both the ship's ops and off ship vendors, and rarely find that much difference between the cost or quality of dives. I have never had a bad dive in any port with any vendor, so I would not worry about your expectations. Just enjoy yourself. All of the ship's excursions include transportation if needed, and the dive equipment (less wetsuit). If you are going to compare pricing, make sure you include equipment rental and taxi costs. Some of your cruise stops have very long port times so you won't have to worry about missing the ship's departure which gives you wide latitude as to who you select as your dive/snorkel op. With short port times, I would usually just use the ship's op for less hassle and stress. One thing not mentioned (unless I missed it) in other responses is the added benefit of using the ship's op to preclude losing your dive/snorkel trip if the ship is late getting into port, or there is a delay getting off the ship for any number of reasons. On more than a few occasions, those of us on the ship's dive excursion got to dive, while the off-ship divers missed their trips because their dive boats had already departed before they were able to disembark. From my wife's standpoint, the biggest advantages of me using the ship's dive op is her not having to worry about whether I am going to miss my dive, and more importantly, not miss the ship departure time. She would not be pleased with me spending a bunch of extra money and days trying to catch up to the ship if I was late for ship departure time.
Edit: When I am on vacation, I try to reduce my stress and aggravation as much as possible.
Edit: When I am on vacation, I try to reduce my stress and aggravation as much as possible.