It's common to see in message boards, forums, and news groups that using water to clean a fretboard can cause problems, or is otherwise advised against. It's not normally explained why, or if it only applies to some types of woods, and normally given as advice in non-formal contexts.
Even when you are not using water to clean the fretboard, it can get wet from cleaning the strings, or other parts of the guitar.
There's a lot more info about oils, but not much about water. Long-term, excess humidity can negatively affect the guitar, but is cleaning the fretboard with a wet rag dangerous for the instrument? Why / why not?