A navigator is the person onboard a ship responsible for the navigation of the vessel. The navigator's responsibilities includes planning the journey, advising the captain (or pilot) while on route, and ensuring that hazards or obstacles are avoided.
A navigator is the member of the crew who is the most knowledgeable about the world as it is crucial for the determination of course and direction when traveling. A good navigator has a skilled understanding of tools such as maps, compasses and sea charts that are able to not only travel through the sea but use the sea to their advantage in whatever direction they are headed. While not typical, many navigators have the knowledge of cartography, giving them the ability not just to read maps but to create their own maps of their journey, taking the knowledge that they pick up and using it to create guides for others from crewmates to other people in other parts of the world to know and understand what they are entering when in a certain area.
While many navigators must have basic knowledge and ability to use the common tools and abilities of their trade, they are also required to pick up on new matters that make traveling across the seas easier and better. In particular, navigators traveling through the Grand Line must gain an accumulation with the Log Pose, the only known compass that can travel through this part of the world. Due to its intricate and delicate nature and its special attributes such as coordinating with an island's magnetic field and the ability to point in directions not normally known for travel (such as the sky or the depths of the ocean), navigators must come to grips and understanding the nature both of this compass and of the world upon which the Grand Line is part of and assist in allowing the rest of the crew to make necessary adjustments in their ship so they can follow the desired path. Without an active navigator, particularly in the Grand Line with an understanding of the Log Pose, journey becomes much more difficult leading to the crew getting lost, like Brook once he revived, or coming across unexpected matters whether or not they are already mapped out.
In One Piece, navigators are the most crucial members of a ship, especially when traveling the Grand Line. Whenever a member of a crew, be it pirates or marines, is seen with a Log Pose tied to their wrists, their position as their respective crew's navigator becomes immediately evident, since Log Poses are one of the most crucial tools of a navigator. A ship is only as good as the navigator that directs it. Otherwise, the ship will be damaged or shipwrecked by unforeseen obstacles. A strong example of this is when the Barto Club is caught in a hail strom and have no idea what to do, because they have no navigator despite being in the New World. Unlike other members of crews, such as captains, doctors or chefs, navigators aren't formally introduced. Characters that are classified as navigators are rare, despite how important they are to the ship.