I recently stayed at the Hale Koa hotel and since this attraction is right next door, I decided to take my 6 year old granddaughter on the premium submarine. Children must be at least 36" tall to board. No exceptions. There are two submarines. The premium is the larger of the two and goes the deepest of the two. I believe it holds 64 people. I bought a special online ticket that said it was one adult and a free child, which I didn't quite understand because it was more expensive than an adult ticket but less than an adult plus a child's ticket. Either way it is usually cheaper to buy on line than if you call or pay at the ticket stand.
You meet at the ticket stand which is at the back of the Hilton Hawaiian Village next to the beach. You walk out on their private pier where they take your tickets and take the obligatory photo which of course they will try sell you on your way back for $45. Not worth it. You board their tender boat which takes you out past the breakwater to meet up with the subs. Both groups go out together. The ride out is fun and sometimes they see whales (we didn't), but the views of Waikiki and Diamond Head are great. Our day was overcast which made the views a little less dramatic and the underwater items a little harder to see. Of course, Atlantis can't do anything about the weather, but you might want to check before you go.
Once you reach your sub, the boat pulls along side and you board. Once inside you take your seat at a very large porthole and they prepare to dive. These are real submarines so getting to go to over 100' below the surface is a great experience. As you dive and began your journey, you will go past manmade reefs, two sunken ships, two sunken airplanes and see the reef. There are lots of fish, star fish and sea turtles. At 100' feet most of the color spectrum is filtered out by the water so you will not see a lot of dramatic colors, Mostly green. My bright red shirt turned purple.
All that being said, my 6 year old thoroughly enjoyed it and it was a fun trip. The narrator was funny and informative and very sweet to the young children. He teased a lot and told great if expectedly corny jokes, but all in good fun.
You then resurface to re-board the tender boat to make the short trip back to the pier. I do highly recommend these at least once.