This attraction is definitely more of a ride/experience than a chance to actually view wildlife in the ocean, despite all the promotional materials that feature an improbable array of colorful fish, sharks, rays, turtles, etc. Unless you're supremely lucky, you won't see that. It was fine for our purposes, but if our child had been older or more savvy, he would have been disappointed. (My son is 4 years old and obsessed with the ocean, and loves being on boats.)
We were picked up at our hotel by a van emblazoned with pictures of the Atlantis submarine and drove around town picking up other guests. Then we were dropped off at a pier on Waikiki Beach near the Hilton Hawaiian Village, where we got our "boarding passes" (pieces of laminated cardboard) and waited for the ferry to come in. A photographer took our photo with Diamond Head as a backdrop, later to offer prints and magnets for "only" 45 dollars. This was the moment that the experience crystallized as Disney Lite for me.
The ferry ride away from shore was enjoyable. There is a concession stand on the boat, but we ignored it and went upstairs for the view. There are also bathrooms on board that you should take advantage of, since there are no bathrooms on the Atlantis submarines. We got great views of Waikiki and Honolulu, and more photos of Diamond Head. The water is a sparkling turquoise that turns to blue as you go into deeper areas. Part of the tour involves explaining the science of why this is.
As we neared the location of the submarine, we were able to watch it emerging to the surface, after which its previous passengers were let off (slowly, as you have to climb up a narrow set of stairs through an open hatch, and even slower as people realized they could ask the crew for photos of themselves with Diamond Head in the background). Once everyone was on board the ferry, new passengers in our group were called to board the submarine. The boarding passes are color coded so that you know when it is your turn.
My 4 year old had no problem getting down the stairs climbing backwards, but I noticed that some older adults and parents with really young children took a while to get up and down - something to think about if anyone in your family/group has any mobility concerns. On the way out, there was some water dripping near the stairwell as well, which could have made it slippery for some people.
The tour itself takes you to five specified locations, which you can follow along with in your "dive guide". It's mainly artificial reefs and deliberately sunken ships and an airplane. The visibility at midday was decent, so whatever was around was easy to see. Doing this tour when it isn't sunny probably wouldn't be as satisfying.
The tour guide was very chatty and made a lot of half-funny remarks, including some jokes at the crowd's expense (like pretending that he saw a whale, then chuckling at people who fell for it). My son just tuned him out - it was way too much talking and the information was sprinkled in between the jokes. Speakers of other languages had headphones that they could listen to, presumably of a pre-recorded tour.
The actual views out of the ship were okay. And I really mean just okay. We saw fish, one random ray, and a sea turtle resting on the sunken ship, but it wasn't a very immersive experience. I think that the promotional materials oversell the view. Supposedly we had a good run and were able to see "a lot", so I'd imagine that other tours take place in which people are rather disappointed.
If you really want to see specific creatures or a large variety of fish, you should go to the aquarium. If you're in it for the boat ride and just being on the sub, this is a slightly overpriced but fun experience. My son wanted to go "again and again" but this is not something I would repeat.