GPS is great, and with world-wide coverage that helps get accurate positioning of vessels. But do you need a backup. The answer is yes, and that would be celestial navigation, its great to learn and it can be a interesting hobby.
Instances of electronic failure, total electric failure, and flooding are often documented. Even battery powered handhelds can be rendered inoperable in these ways. Batteries can run down, spares can be lost. The GPS system itself is not guaranteed.
GPS will track your boat and steer your boat. Some say it will even take your boat across the ocean for you. Without establishing a discipline, one's navigational skills will be lost. The key to navigation is the time-proven DR track. It should be maintained and updated with fixes, whether electronic or celestial. This gives information about the current and leeway, steering and compass errors.
Who can contemplate an 18th century brass and ebony sextant and not wonder what it was like to peer through it at the heavens, and bring an evening star down to a twilight horizon from the deck of a tall ship? To sense the approval of those who witness this magic-like prowess. To triumph at a land-fall well predicted? To know that they can navigate any ocean with no help from anyone?
Fun is doing something that is both easy and difficult. Easy to get started with, but having enough challenage that mastery does not come easy.
What could be easier than reckoning the longitude by simply observing the time of sunrise or sunset, or steering by a star? Almost as easy is the finding of latitude at noon. But how about identifying the navigational stars? Recognizing the planets? Accounting for the parallax of the moon? More experienced celestial navigators can use an unknown star shot through a hole in the overcast, shoot planets in broad daylight, predict sunrise underway, and calculate great circle distances.
Being familiar with the night sky is like having a giant roadmap overhead. One star may lead to another and before you know it you can identify 12 - 20 navigational stars. It is really cool to beable to take a sight of Sun, Moon, Stars, or Planets and get your location. People enjoy using sextants, even from a backyard with an artificial horizon. It's one of the few nautical activities that can be done without a boat, or even being on the water.
