I know this is a bit off topic, but I have concerns with staff of flowing water give ability haste.
Not only is haste a powerful invisible stat to opponents, but I feel that the item's power will vary widely by rank and communication level. Saving your abilities until you can get prebuffed is way stronger with communication, but making tracking ability cooldowns way harder on the other side. Moreover, it makes optimal play way harder when you don't have quick and easy communication methods available.
IIRC in league's early days shurelia's gave AOE CDR (5%) and it was removed for bringing invisible power to an invisible stat.
Finally I am a bit worried about it augmenting champion's play patterns too much. It feels like a general design principle in league is: legendary items shouldn't change ability sequences too much. You might pop a cooldown to stop steraks from procing, but for the most part (other than zhonya's) you play the same way before or after an item. I'm feel like the ability haste could incentivize very augmented play a bit too much.
Edit: one more concern I have is that compared to the old passive and ardent censor, it is way easier for the new staff to have it's passive be completely useless in an aggressive fight