As someone who is more than good enough at fish scout to play him in a serious game and not be dead weight, I made the decision to compile some tips for this completely serious, non-joke subclass.
(its a meme it's just a pretty powerful one in the right hands)
also that isn't my loadout i found it on google images

-By my definition, it is okay to use mad milk or cleaver as fish scout. They aren't guns, that's what matters. Use the cleaver if its pyro hell, so you have any way to damage them (or just don't go fish scout when there's a lot of pyros, dummy) and use the mad milk to make dealing with things like heavies and demomen easier.
-You may not think it, but heavies are actually mostly on neutral ground with the fish scout. if you can get behind a heavy, and circle around him well enough to confuse him, you'll quickly shred him quickly. But if you try to face him without a plan, i mean, what are you even expecting to happen?
-See pyro, run (or throw cleaver/milk then run)
-As a fish scout, your one biggest job is surprisingly enough to manage the flank. Your fast-swinging fish of doom will weaken any enemy even if you die first, leaving them for dead as they either run into enemy territory or retreat. You can also quickly take care of their medic.
-It may seem attractive to be the run in funny whack man, but fish scout is by all means a defensive subclass, if your seriously trying. Like I said, you are great at weakening enemies trying to flank, and because of this, your engineers and medics will always be grateful for you being their first line of defense instead of their deadweight line of "offense."
-You can cap as fish scout, but you NEED to know the situation before trying to. Enemy pyro dead? Enemy engineer's sentry just got downed? Go for it. You don't have a medic and they have one that is pocketing a pyro? ♥♥♥♥ outta there.
-Along with taking care of the enemy medic, make sure to treat your medic like gold as well. Treat them like that one australium you really want. If your medic is in danger and low on health, he comes first as a fish scout. Way more than with classes like demo. Don't just be his personal bodyguard, but always keep him in mind.
-You are always at odds with a spy. It's part of your job as a defense class to take care of spies, and as the first line of defense most of the time, you're also a big target. Be wary of this when you look away from that pyro walking towards your intel for a second.

This has been fish scout tips. Now go out there and teach those ignorant heavies and cocky medics their place.