They're having an underwater tea party. An imaginary tea party is a common playtime acivity among kids; Johnny and Cleo are simply having it in a pool. Says Stephen Dorff who plays Johnny (about the Chateau Marmont hotel):
When you stay in a hotel for that long, and you have friends come join you, you kind of host the party every night, so I'm paying for everything — and ... it can add up every time you have a coffee. It was a good lesson to learn. I then ultimately had my 21st-birthday party there, which ended in a big ... synchronized-swimming party in the pool, which ultimately [in Somewhere] I do my underwater tea party with [onscreen daughter] Elle Fanning in, so there's been all these moments there."
Here, for example, is Cloe when she mimes sipping from a pretend tea cup:
