Compasses were traditionally tattooed on sailors and boating enthusiasts. They symbolise not only been at sea but also finding direction in your life or a particular journey you embarked on. Nowadays they are popular for aesthetic value as well as for people such as backpackers who have done a lot of traveling and want a compass design. Some will even have the compass face the direction of their home from where they have moved to so as to have a reminder of where they came from.
Since the Disney movie The Little Mermaid, the mermaid form has been a popular style for both costumes and tattoos alike. Above you can see a cool example of an under the sea style theme on the forearm. Silhouetted styles are generally safe choices as the thicker line work makes them last longer and less intricate line work that can potentially be messed up or fade over time.