The last monorail stop is Atlantis, where there are two separate tickets: Aquaventure Waterpark, open 10:00 to 18:00, and Lost World Aquarium, open 11:00 to 20:00 and indoors. The waterpark is the outdoor one and has a 1.2 m rule on some rides; the aquarium is the indoor one, and is what we still sell under its old name, The Lost Chambers.
Which of the two to choose, and whether the ticket that covers both is worth its price, is a question of its own: our Aquaventure and Lost World Aquarium guide answers it, and both tickets are below.