Cast time makes Aqua Prison take ~0.875 seconds from casting to landing regardless of distance.
Since the area of effect has a ~162 units radius, an enemy unit with 185.14 or less movement speed (325 affected by a 53.8% slow, due to the low-end movement speed soft cap) would find it impossible to escape.
As a result it might be necessary to have another source of crowd control in order for Aqua Prison to land. A fully-ranked Tidecaller's Blessing powered by 376 AP would be able to slow for the amount above, alhough it is unlikely an enemy has not purchased movement speed by that time.
Ebb and Flow
COST: 70 / 85 / 100 / 115 / 130 mana
Active:Nami unleashes a stream of water onto the target champion or herself, which then bounces to nearby unaffected champions up to twice, alternating between enemies, allies and herself.
Ebb and Flowheals allies and deals magic damage to enemies, with each bounce being 85% (+ 7.5% per 100 AP) as effective as the previous one.