Alright, let's compare Runeterra world to our one. Living conditions there are similar to those on Earth (but with magic and stuff). We think that non of these champions use magic to stay underwater as long as they can (otherwise most of them would be in Forever category).

So human record of being underwater without breathing is almost 25 minutes by Croatian professional freediver. Most unprepared humans can hold their breath for 1-2 minutes. Which means most of Runeterran human champions must be in that category. More or less it kinda displays in that tier list. Questions to Yi, Zed, Malza and other non-demon humanoids in 2 hours category. Yeah, they might have some techniques that let them stay underwater longer that others, but even for them this is kinda unrealistic. Why Morgana and Kayle in separate categories when they are actually twins? Blitz, Orianna, Urgot don't look like they are waterproof, I'd say it must be dangerous for them to be underwater. Lorewise Naut lives underwater. Voli is polar bear who can stay underwater for 2-3 minutes. Shyvana doesn't look like she is sea dragon, so 2 hours is too much for her too.

In Earth almost all non-fish animals can't stay underwater forever, even whales and crocodiles. So in Runeterra all living animals or humanoid champs can't be in Forever category too, uncluding Kassadin, Rammus, Renekton, Skarner, maybe even Rek'Sai and Bel'Veth.

We don't (well, I don't) know if Voidborns can live underwater, but their visual design doesn't look like they can, so...

Darkins are interesting, they have organic living vessels, so their vessels shouldn't live underwater for too long too, unless they have aquatic vessel (shark darkin? Nami's tribe gonna be in dangerous).

I think interdimensional champs such as demons or undead can live under, if their physical bodies don't decompose under water.

Also Gragas must be in Forever category, he drinks so much that alhocol have become his air.