Nami's ultimate can come in very handy for cutting off escapes.
Playing Against Nami
Aqua Prison is a very powerful ability, but has a long cooldown. Take advantage of the situation if Nami misses this ability.
Tidal Wave has very long range but travels slowly, it is easy to juke if you see it coming.
Try to avoid fighting an opponent affected by Tidecaller's Blessing. It lasts a short duration, so waiting it out can help you greatly.
Nami is an offensive enchanter that delivers augments and crowd-control capabilities for her teammates. Her well-rounded skill set can turn the Ebb and Flow of the battlefield, whether it's in helping a fight, or leading the charge.
Ebb and Flow is a fickle ability. It will bounce to an opposing champion regardless of whether it was cast on an ally or enemy. So if you're looking to trade with this ability, keep in mind of this commutative property. Instead of hitting the enemy and waiting for the rebound, you could cast it on yourself and back away as it bounces to the them.
To maximize the amount of bounces out of casting this ability, a good rule of thumb is to throw it on the team with more members present.
Ardent Censer is a staple item that provides the essential stats needed for Nami's playstyle. Start sticking with your allies by the time you get this item to utilize its full benefits.
Redemption is another core item that doubles down by providing a healing field should you need widespread sustain.
Mikael's Crucible's immense stats are really good if you battle oppressive engages, but note that it has a Harmony passive, meaning that having Athene's Unholy Grail at the same time will be problematic.
Since Nami has a well rounded kit, battling her efforts usually comes down to specialized brute force.
Champions such as Brand and Zyra dish out challenging amounts of poke damage for Nami to keep up, and play decently during engages.
Champions such as Braum and Yasuo can counter Nami's spells for a clear shot at retaliation.