Lead boat dictates the line.
There is a bit of a fine line here. If the lead boat doesn't actually need to move over (they could hold their line and have a clean course), then it is dirty racing. I think the lead in that vid was Tim Brabants, he was being a little dirty - he left safe room for the boat on his left, but forced them off his wash. That's ok. If he'd forced them into the buoy, not ok.
I'm not sure how precise the rules are in kayak racing. Yacht racing specifies this according to the exact relative boat position.
In my race, the rules said we had to pass to the right of a buoy. Group of 5, I was on the left. I had 4 choices:
- Ram the buoy.
- Go left of the buoy and take a time penalty.
- Speed up before reaching buoy and edge right.
- Drop back and move right.
I chose the third, put more than half my boat in front of the other boats. Boat to my right tried to move up, we clashed paddles, he was 20cm to my right. I put my hand on his bow and gently moved him away before we both ended up in the water.
I didn't want to drop back because the group was going a little slowly for my liking. If I'd dropped behind them, I would have had to push through their wash to get past (wasted energy).