The vessel is Atlantis XII. The operator's own expedition page calls it a pressurized tourist submarine with more than 26 side portholes and a bow viewport. You check in at the welcome center on the hotel-zone coast near the south cruise piers, take a short transfer boat, climb down the hatch, and run a narrated route over Chankanaab reef, the Felipe Xicotencatl wreck, and the wall drop. The outing is listed at about two hours. The underwater piece is the middle of that, commonly around 40 to 45 minutes. The rest is briefing, tender, and the ride back. You do not swim. You do not snorkel. You sit, look through a porthole, and listen.