There is indeed a change in 2.4 intended to offset some of the mana regen from drinking away from the very beginning of the duration.
However, we specifically made sure the water would not change the amount of mana regenerated by players who drink between pulls or during gauntlet runs. You should see little to no difference in the amount of mana yielded if you sit to drink for at least 4 seconds, which most players do.
So right now you get a tick roughly every two seconds. After the change the first tick will give you nothing, second tick will give you normal tick+66% extra, the third tick normal+33% extra, and then it levels back off after that. So, at the ~6 second mark (or after three ticks) you've made as much as you would have before.
The change has to be consistent across all water as this fix applies to Arenas, battlegrounds, and world PvP, which will have specific emphasis in the upcoming expansion.