Question about "rules"
Rules are what the community wants them to be. They aren’t really following any logic.
I’m pretty the end boss is more dangerous on land than water so it’s allowed to give people a choice. If it can somehow be abused it will be disallowed.
Less important and not my question but still interesting: why killing last boss of hotw p3 NOT in water is, ehm, legit?
Pulling the boss to be able to fight him above water is a safer way to kill it (at least for 3 man). In a 5 man group, it would be a waste of time since you have to first pull him and wait until he will los. Also for the spot you saw in the tournament, if the pull isn’t done properly, it will be effectively above water and just kitten the team, so you take a huge risk by pulling the boss to that spot.