Quote from theknightazrael224 >>

Can you cast it on your self as well

 

 Yes, you can

Harry Potter did.. 😏

For spells like this unless it says otherwise then yes, after all you are a willing creature within range that you can see.

🤓

- This can in a way cause suffocation damage with some strategy. -

For example. have dispel magic prepared as well as water breathing. Affect a target with water breathing. It would either try to use the water to its advantage or if you had drawn the target's aggro, then it would follow you in. 

All you need to do. is get down in a bit of water deep enough. and teleport up as a bonus action and cast dispel magic on the target, to remove water breathing. if the target cannot normally survive underwater, wouldn't it continually take suffocation damage?

That and possibly, difficult terrain. 

does the spell transfer over to wild shape if you cast it before you wild shape?

It should.

Can a person under the water breathing spell use their sense of smell underwater?

Quote from Brelsfordj1 >>

Can a person under the water breathing spell use their sense of smell underwater?

 I would guess that any sense of smell would be overwhelmed by making sense of an entirely new set of smells for quite some time, unless you spent a lot of time underwater this way. Maybe strong things would be obvious, but I don't think I'd allow for a lot of subtlety. For example, I'd say someone might notice the smell of sulfur in the water about the same time they noticed it starting to get warmer as they approached a volcanic vent. If you're doing a perception check with the player(s) based on smell underwater, I'd personally advise either raising the difficulty of any not accustomed to the environment, or perhaps imposing disadvantage.

Quote from KnightofSvea >>

Harry Potter did.. 😏

 Harry Potter used Gillyweed. 

Unless it specifically states otherwise, all spells that say "up to X creatures" or "a creature you can see" can be cast on yourself. You can see yourself and you can declare yourself willing.

Its spells that specifically say the range being self or only yourself that have this limit.