I dove on three of the four stops on our last cruise and also am new to diving. All of those 3 that we dove, had a long port time and the ship was docked (no tenders to wait on to get to port), so they were really easy to ensure that we would have ample time. It looks like your ship will have a long time in port on a couple of days, so those stops should be pretty easy to set up a dive with a LDS.
How I picked the LDS to use? I did a lot of searching on the internet and reading up on them. Then I emailed them several times to ensure that they would be able to ensure that I would be able to dive with them and make it back to the ship in time. The shop in Roatan even had a cab waiting at the dock for us.
If you don't want to risk getting back late to the ship, you can always use the cruise ships dive excursions, but I was scared they would be cattle boats and certainly didn't want that.
In Costa Maya, my young kids and I swam with dolphins first and then my older son and I went on a 2 tank dive. We were ensured that we had ample time by the dive shop, and it worked out that we did. BUT, I told the DM before I first dive, that if we were running late, that I would pull the plug on the second dive if time was an issue. He told me not to worry that dive boat captain was also the pilot that the cruise ship used to navigate out of the port, so they weren't going anywhere without us. :cool2:
I saved 30-50 bucks on my son and my dives EACH by using LDS's instead of the the cruise lines, and I think we had a much better experience than we would have. Just realize that the only way to ENSURE you won't get left is to use the cruise excursions. The boat will wait for thier excursions, but they won't wait for anything not booked through them.
Also, when trying to book the dophin swim and dive excursions in Costa Maya, the dolphin swim had to be booked through the ship. When I booked them early, the guy with the cruise line told me that I couldn't do both dolphin swim and dive. They couldn't work around both excursions. By booking the dive through an LDS, we did both and it went great.
Good luck, and I know you will have a blast. It is truly incredible for beginners to be able to dive in those spots. I kept telling myself to remember to BREATH!
As far as extreme, just let the dive shop know what level/comfort you are, and they should work well with you. I know ours did.