The Water Challenge is the most annoying task you'll ever have to do in Asterix XXL 2, not because it's hard, but because the Mario wannabes prevent you from attacking with their constant knockback. Fortunately, there is a quick and easy way around this issue.
Before starting, if you haven't already, make sure that you've beaten the game and unlocked at least the Tornado Combo (also known as Belenos' Rage) which can be purchased from a store located at Lucksore - Hall of Illusions and it costs 1500 Roman helmets. It's right next to the fast travel.
Once you've met these conditions, set the game on easy (just in case; enemy attacks are less frequent on lower difficulty and you want to avoid receiving knockback) and go to Venitia - Carnival Square. Clear the challenge room from the three patrolling Romans first and then go back to the potion lying beneath the plaza.
Pick up the potion and rush to the challenge room in order to quickly start the mission. Once the challenge has been activated, use the tornado move by holding down L1/LB and input the combo from the first image (pending on your controller). Once activated, repeatedly tap the prompt to fill out your power bar and within a few seconds you'll have cleared the challenge with ease.