What is the difference between a fishpole and a boom pole?
To be honest, man, I've never heard anyone use the term 'fishpole' in regards to video/film production. Recording audio with a microphone that is connected to an extendable rod is called booming. There are people in the film industry who make a living solely from being a boom swinger. 
I think fishpole is just another word used for the same thing, although if someone referred to a boom pole as a fishpole onset they would probably get laughed at. 

So yeah, unless you are getting filmmaking mixed up with the honorable profession of capturing sea-dwelling animals, I'd say there is no difference between the two.