A clean physical-omen Chekhov tell: Oda has a character state the mechanism out loud (deeper ebb → bigger surge) and shows the record-deep drain, so the reader knows the return flood will be the worst on record before it hits — pure setup for the arc’s flood-disaster climax. Distinct from the train-service-halt clock (Aqua Laguna Last-Train Service-Halt Clock Water Seven) and the sole-exit-funnel structure (Aqua Laguna Tonight And Sea Train Sole Exit Funnel To Enies Lobby) — this is the ebb-depth → wave-size beat specifically.