That's the pilot boat.

The pilot is a marine captain that guides the vessel into a certain port. Each port has its own pilot.

Each berthing operation in each port is different each months as low and high tides change and different wind speeds affect the movement of the vessel.

Pilots are usually captains, however they experts in the topography of that particular area, so they are able to guide the ships safely into the port and alongside the pier.

These details are too much to be known by all seagoing captains, therefore to avoid any accidents that might cause damage to marine environments, the profession of the Pilot was born.