Use AnchorSentinel to provide navigation services and protect your boat.
AnchorSentinel is a nautical program with three primary purposes: It provides an accurate position, speed and bearing (true and magnetic) while underway, it stands sentinel against anchor drag while at anchor, and it provides an arrival alarm for a specified travel distance. AnchorSentinel relies on the fact that most Android devices have a built-in GPS receiver able to determine its position within a few meters, adequate accuracy for AnchorSentinel's purpose.
For boaters who haven't been in dangerous situations, or who don't anchor very much, I should say that an anchor watch is a very good idea, and an Android device is an ideal platform for this task because it requires very little power, it operates independently of the boat's primary navigation and power systems, and it provides a secondary source of accurate position information in an emergency.