I have been traveling down to Mexico on spearfishing trips for over 20 years and have never encountered a problem with bringing spearguns in or out the country.
I normally transport them in a tube or ski bag, so as not to draw attention, but have not been bothered even when inspected.
Until today, that is!
I was flying out of Hermosillo back to the US and after being inspected, was asked to follow Mexican customs agents upstairs to a room where they unpacked the guns, then started filling out all sorts of forms.
The forms had space for a fishing permit/license number to be put down.
Fortunately, I did have one with me.
After some stamping of paperwork and signing of forms, I was permited to check my gun bag and board the plane, only just in time to make the flight.
It was hardly a "Midnight Express" experience, but I do wonder what would had happened to my spearguns if I hadn't had the fishing license.
Those of you traveling with spearguns might want to invest in one, just in case.