Possibly the maritime navigators most essential tool! It has been in use since at least 850ad for navigation, but its priciples are said to have been descorvered as early as 1000BC by historians. This part of the project should be split in to two catagorys, an intergrated digital compas and an old school alalogue compass should there be an electronics failier.
The best type of analogue compass to use in marine aplications is a liquid filled compass, or bearing compass.
LSM303 and arduio, with display out and serial conection to main computer/other equipment.