@mr-f4hu
The ladder is clearly doable, it is the jump on and off and he can do it that often due to going further up the ladder while you reload which is clearly visible. The hit-registration really needs fixing.
The water movement I bet is based on their DPI settings, though I have never seen it that extreme as it looks like he changes direction really quick while swimming and most likely jittery due to latency and well being quicker than 'normal' doubt he aims/plays with that DPI setting and only uses it for dodge movements. Doubt it is hacking, but it is try-harding and might be something the devs could place some boundaries on, because there should be no reason for a pirate to be able to turn that fast.
@Talriaen The fact is that due to not having the gamer tag in question or any personal information being shown by the OP this should be above board and not an issue to show game-play to question: How did this person do it or is this a hack? He even properly disclosed the verbal nature of the clip and making it an external link. No witch-hunt going on, because for that you would need to point out the witch?