@ShiroRyu13
Ok.... I really hope you're not reporting people for this stuff. Almost everything you listed is easily done, I do it all the time. You're simply being beat by people better than you.
If they can hit a head-shot while jumping and evading their character, multiple times, they are using a hack. -- No... not at all, this is one of the easiest things to do. There's no headshots in this game, there's no reason to even aim at the head.
If they can hit you the second you peak out from cover to confirm their location, again multiple times, they are using a hack. -- You're mad because someone knows your location and is waiting for you to peak from behind it and shoot you? C'mon bro.
If those same characters can snipe you underwater, with a pistol, from a moving Sloop or Galleon, without hitting the gunpowder you are carrying, multiple times, then they are using the hack. -- Pistol is my favorite weapon, and you best believe you're getting "sniped" with it from my moving galleon when you're swimming.
When you're under their boat and they know exactly which side of the boat you are on, from the deck of their boat, and instantly 1-shot you with a pistol, multiple times, they are using the hack. -- Headphones dude.... headphones. A pistol won't 1 shot you either unless you're already injured to a point, or you were hit a few times at once from different people.
If they are able to 1-shot another pirate at the wheel, on a different moving boat, while they are jumping and evading another pirate, multiple times, they are using an abuse. -- This is literally the only thing you listed that would be DIFFICULT to do, but not impossible.
Just like I recommend to my friends when we're playing games like Battlefield or Rainbow Six, record the "offender" and rewatch the video yourself. Usually you can see where YOU screwed up, or they OUTPLAYED you. Maybe it will save you some frustration and grief.