There's a reason for it, it's not just "made up".

Salt water is considered "mildly corrosive" - aka may rust things. Sure, most of the metal in a plane is aluminum (and now carbon composite in the 787 and A350), but not all of it.

Grab yourself some water, make a high salt solution and dump it somewhere critical and you could potentially down the road compromise aircraft integrity due to galvanic corrosion if other metals are present.


Hawaiian Airlines specifically mentions salt water:
http://hawaiianair.custhelp.com/app/...check-in-items

More info about galvanic corrosion, specifically relating to aluminum in salt water:
http://www.boatingmag.com/protecting...ater-corrosion

As a tangent, Mercury is even more dangerous to aluminum: