Floor, Story
Look at a building and compare the white parts. Each of those white parts is a floor. Multi-story buildings have tons of different floors that you can see from the outside.
All buildings have white marks that you can compare with the rest to see where the floors are.
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You need to use your ability to tell where floors are from the outside of the building to find the story that lets kayaks (かい) in. You're coming from the side of the building surrounded by water, so you need to find the kayak floor to get in.
Picture this building and each of its floors. There should be one floor in particular with a waterfall coming out of it that you can climb with your kayak.