Quite apart from the local risks of defective equipment, battery failure, generator failure and short-circuits to electronic navigation equipment caused by flooding, there is a very real global risk that the GPS could be destroyed.
During periods of increased solar activity, massive amounts of material erupt from the Sun. These eruptions are known as coronal mass ejections and when they impact with the Earth they cause disturbances to its magnetic field known as magnetic storms. Major magnetic storms have been known to destroy electricity grids; shut down the Internet, blank out communications networks and wipe out satellite systems (including the global positioning system). There is scientific evidence to suggest that increasing solar activity will cause major magnetic disturbances in the near future and NASA has issued a major solar storm warning for 2012.
Couple the danger of solar storms with that posed by cyber terrorists who could block GPS signals at any time, then it can easily be seen that navigators who rely solely on electronic navigation systems could be faced with serious problems.
How could seafarers navigate the oceans if the global positioning system (GPS) failed? The answer is, they could revert to the tried and tested art of astro navigation.
Astro Navigation Demystified 
More than interesting, I have been doing a degree in Yacht Operations and used this very subject for my special project on the second year, the amount of information I now have on this problem is mounting fast. I started from a position of GNSS were the best we have, now I will not ever rely on GPS alone. GNSS denial, spoofing and as mentioned terror attacks are only a small sample of the problems with the system that I have found, I now own a long term Almanac, a sextant and also I have made a sextant as well as a kamal, I also calibrate my fingers and hands at arms length. My long term almanac is not vulnerable to mass coronal ejections, terror attacks or even being switched off, it runs until 2050, it is now my firm opinion that it will see the end of GPS and still help me navigate.
Thank you; your feedback is very much appreciated. Good luck with your research and with your degree.
Jack