What do David and Marita do while alone in their cove after he finishes his first short story?

When David and Marita are in their secret cove in Chapter 19 they discuss the problem of Catherine. She will not see a doctor, and they feel she is increasingly becoming insane. In an act of total trust, Marita asks David to dive from the rocks past her shoulder, as close as he can. He is frightened when he almost hits her, but she loves it.