Fishermen and scuba divers are, in practice, generally not going to be busted by police for having a normal-size knife in Japan. Of course, this is the problem with the legal strategy of “make it illegal but give enforcement discretion to the police” - it adds a lot of uncertainty to what you are actually allowed to do, it allows the government to convict many normal people of a crime if they want to, and it gives the police discretion to get people in trouble for arbitrary reasons.