As you probably gathered from all the water metaphors and the use of chapter intros that focus on an ocean landscape at different stages of the day/tide, water is the central metaphor in this novel.
Woolf's obsession with water, fluidity, and flow "seeps" into every narrative pore, including the stream-of-consciousness style and the similes and metaphors the characters use to describe themselves and others (and particularly their emotions).
With the chapter intros, Woolf draws connections between the stages of the human life and the 24-hour cycle of ocean tides, suggesting a structural similarity between the two. Because the novel presents us with a variety of waves—from literal ocean currents to character narrations that ebb and flow like the tide—The Waves seems like a pretty appropriate title.