On water slides where you sit on the inflated innertubes, you have to climb up the stairs to start the ride. Why not get shot up by a column of water instead?
I envision something like the straw used in plastic bottles of mouthwash (well, in the US anyway).
The person waiting to go up enters
a tall room like an elevator shaft. This room has a hard outer wall and a soft inner wall, which is decorated with underwater scenes. The space within the soft inner wall contains a deep pool, and a transparent tube about 2m in diameter, like a giant straw, sticking out of the pool. The straw/inner column is also filled with water. The straw sticks out of the ceiling of this tall shaft-like room. (I guess an air-lock type system is necessary here to get in the room.) The person enters the room at surface level of the pool,and sits on an innertube, and grabs the handles tightly.
A ride attendant (on intercom) now announces that the person should take a very deep breath, hang on tight, and keep their elbows and knees tucked in. The innertube (plus person) gets sucked underwater and manouvered right under the inner column of water using amusement park ride technology -- there was a pulley system with a release lock attached to the inner tube somewhere. And when the person hits the exact spot underneath, the attendant hits the switch!
The outer, soft walls push inwards, compressing the air column, pushing the surface of the water down. At the same time, the inner tube is released from the hook holding it in place. Whoosh! The person is sucked into the tube, and enjoys a fleeting view of all the fish etc. decorating the soft walls. Suddenly he pops out into a different pool, on the other side of the ceiling, like being squirted out of a straw.
Plop, the person's in a shallow pool of water, where s/he bobbles gently, getting oriented again, perhaps climbing out of the water and onto the tube again if s/he arrived upside-down. But the person has to hurry, because slowly but unavoidably, the tube is being carried by the flow of water in the pool to the entrance of the water slide! Two rides in one!