This is mainly applicable to an office environment, particularly one with cubicles. The clock shark will walk around inspecting employee attendance. Their heads are visible over the tops of the cubicles and resembles a shark swimming around the office.
The clock shark themselves typically produce a questionable quantity of work as they spend the majority of their productive hours monitoring the attendance of others.
Employee 2: I can't, my boss is a clock shark. He'll be swimming around at about 4 o'clock to see if I'm still keeping my chair warm. I'll see you after 5.