HUCKLE: As far as we know, the sea was calm and empty.

THE DOCTOR: It may be calm, but it’s never empty.

Doctor Who (1963-1989) British science fiction television series, original run (BBC)

13×01 “Terror of the Zygons,” Part 1 (1975-08-30) [w. Robert Banks Stewart]
    (Source)

(Source (Video); dialog confirmed)