This is a long one with a quest string.

First find Finlor the Sailor in Britain

Finlor the Sailor

He will then give you a magical chest to keep everything in.

Mystical Chest

When you have completed his quest, return the chest to Finlor who will give you a Mystical Sextant.

Mystical Sextant

This can be used when on board ships to control the movement of the ship

Using Mystical sextant on a ship