I remember being taught as a kid to look and see how much red was exposed on the hulls of bulk freighters to tell if they were coming or going. Lots of red exposed? That's an empty ship coming into harbour. Not much red exposed? That ship is laden down and ready to leave.
(Put your crab traps down near the ships ready to leave, they probably just illegally dumped their sewage so the crabs are swarming around them.

)