If you need a quick escape from a melee champion who is already autoattacking you, try casting Aqua Prison right on yourself, as it will be extremely hard to dodge and will also apply Surging Tides to speed your escape while the enemy is stunned.
Ebb and Flow is great for casting in a 2v2 fight in bot lane. It damages your enemies, heals an ally and activates your passive Surging Tides, which will provide a significant advantage.
Ebb and Flow bounces between allied and enemy champions and doesn't hit the same target twice, depending on which side you cast the ability on.
If cast on an allied champion, the spell will hit as follows: ally → enemy → ally. If cast on an enemy champion, the spell will hit as follows: enemy → ally → enemy. If there is no appropriate unit for the middle bounce effect, such as if no enemy unit is around in an ally → enemy → ally bounce, only the initial cast will occur.
This ability can deal a lot of damage if quickly combined with Aqua Prison and followed by Tidal Wave, this burst is somewhat near to that of a mage character and can kill fragile opponents if you have a lot of AP
The bounce range of Ebb and Flow is larger than the cast range, so sometimes casting Ebb and Flow on yourselves can achieve longer range than casting Ebb and Flow straight on enemy.
Tidecaller's Blessing works great with skillshot champions like Corki and Ezreal, because of their great poking potential and how they rely on spamming basic attacks between skillshots. It's also extremely effective on champions with powerful auto-attack modifiers such as Draven or Kog'Maw to create powerful harassment which is more difficult to escape from.
Nami is a squishy champion, so be careful not to let yourself get caught alone. Positioning is always important.
Care should be taken when utilizing and choosing targets for Champion abilities:
Aqua Prison Offensively, should be cast on squishy targets, like an AD carry or a support, unless they have high mobility or can easily dodge it (like Ezreal or Vayne). In these cases, you should instead aim at a target with a lower chance of dodging it. Defensively, it's (more reliably) useful to peel for your carries after they've been jumped at by someone with a gap closer, like Pantheon or Vi, or interrupt a channeled ability like Death LotusKatarina or Piercing ArrowVarus.
When prioritizing healing, Ebb and Flow should be cast on allies: this will potentially heal two allies or yourself as well. On the other hand, when harassing, prioritizing enemies may result in two enemies taking damage while healing one ally or yourself.
Tidecaller's Blessing damage increases with your AP, not the allied champion you cast it onto. Unlike most other attack steroids, there is no need to choose a high damage or attack speed target. As long as your target attacks three times in six seconds, you get the full effect of the skill. A slower attack speed target may even be able to apply the slow longer. It will, however, benefit from the allied champion's magic penetration.
It will also work with champions that are using on-hit builds like Teemo and with abilities that apply on-hit effects like ParrrleyGangplank.
Casting Tidecaller's Blessing is usually the better option than Ebb and Flow because it consumes less mana and is instant cast. Ebb and Flow may heal you or your allies, but it requires a very short channeling time, which could allow your chaser to catch up.
Rylai's Crystal Scepter is a great item if you want to maximize your slow effectiveness on your offensive skills. Both Ebb and Flow and Tidal Wave can benefit from this very well, since first skill can bounce to enemies and apply full slow and your ultimate can apply additional slow. Beside that you will gain additional HP that can stack effectively with your Sightstone, and portion of AP for even more damage with your Tidecaller's Blessing.
Like all squishy champions, Nami can be countered and easily targeted by assassin champions with stealth like: Evelynn, Shaco and Teemo and champions with gap closers like Fiora, Jax and Xin Zhao.
Keep in mind, if she realizes you are about to close the gap, she may use Aqua Prison on herself. Even if you apply a stun or silence, there is a short delay on cast so a skilled Nami will make a tower dive a regretful decision.
Aqua Prison and Tidal Wave are both fairly easy to sidestep, but be careful when you are fighting in the jungle or being chased, as this limits where you can go in order to dodge both abilities.
Use any item that grants Tenacity. It will render most of her crowd control useless.