I was just told by a friend that I should learn celestial
navigation before sailing around the world because
GPS devices don't work that well in the middle of the
ocean and can malfunction if they get wet, etc.

Does my friend have a point even if I intend to bring
3 independent GPS systems with me ?

How about if I just get a cheap sextant and the book
"Celestial Navigation When Your GPS Fails" and
bring them with me ?

TIA