Leona

Leona is just annoying, especially vs Nami. Once she hits level 6, it will be quite hard vs Leona. Though you can easily out trade Leona in early game (again, before 6). If she tries to engage, go for aqua prison on the adc - if you cannot get the adc, use your q immediately on Leona. This may be able to guarantee a kill on Leona, and if not; she will most likely be backing out with low health. You can also be able to AA Leona a lot easier since you have that range advantage!!

Rell

I just hate Rell in general when I am playing Nami, her cc is outright annoying. Though, you can easily outpoke her. Especially if she jumps in for an engage, you can aqua prison her then AA. Also, do not be scared to AA Rell at times either, but do your best to play back vs her and pick safe engages.

Pyke

Another extreme match up for Nami is Pyke, his movement speed will be better than yours, and it will be hard to out poke him since he can passively heal. The best way to play vs a Pyke is to dodge and play back while trying to exchange smart trades with Pyke. Usually vs a Pyke, I would recommend getting mobility boots asap.

Nautilus

Another hook champion. Play very passively against a Naut, and like Blitz; play behind the minion wave if possible. Stay out of the range of dredge line.

Thresh

Thresh isn't as bad as extreme threats could go, though he is still a major counter to Nami in my opinion. You have probably read this already, but dodge the hook! Stay out of the range of his death sentence and flay; considering his flay just adds an extra threat to the lane.

Rakan

Rakan is pure annoyance - especially when you're playing Nami. You need to time your bubbles and ult. Though Rakan's laning isn't strong, he can still set up pretty decent engages vs a champ like Nami.

Alistar

Very aggressive support champion, and also can set up decent engages for the enemy adc. Before level 2, harass him as much as you can with W and AA.

Swain

This might just be personal opinion when it comes to a threat and maybe just a personal threat when it comes to my Nami, but I completely dislike Swain when playing Nami. His grab and pull are very threatening, and after level 6 he will deal more and more damage. All you can do is dodge, dodge, dodge. Harass him as much as you can before laning phase is over.

Caitlyn

Caitlyn's range is very annoying, and she can out poke tremendously. My advice is to harass her with your W and AA, while still being able to set up decent engages with your Q / or if it comes down to her support setting up an engage, try to disengage her from the fight with your Q.

Varus

Varus's poke is good, you want to do your best to poke him back. If he is fed, get Mikael's Blessing - especially for his ult.

Brand

Brand does a lot of damage. Set up engages as much as you can for your ADC, and focus him in a 2v2.

Maokai

He's not really played as a support anymore, but I have seen him occasionally. Watch out for his sapling toss, because they can easily burst you down and he has the upper advantage with it to harass you. Try to out poke or trade evenly as much as you can.

Tahm Kench

Just like Maokai, I do not see the catfish often in the support role anymore. Just try to stay back from him, and harass him since you have a far more better range advantage.

Morgana

Her black shield in my opinion is really the only threat of a Morgana vs Nami. You can easily poke out Morg with W, AA, and Q. Just watch out for her roots.

Zilean

Annoying, he has cc - but if you can dodge his stun, you can easily trade with Zil.

Seraphine

I generally hate Seraphine as a support, but you can easily out trade her after you wait out her E. You have a better engage advantage with aqua prison.

Lux

Dodge her q's, engage as much as you can, and poke her whenever you're able to.

Yuumi

You can easily out heal Yuumi early game, especially with her mana restriction. Just out poke her when she is dethatched from her adc.

Janna

Lulu

Soraka

Taric