They are swimming in/on/at the pool?

Hello,

They are swimming in/on/at the pool. Which one is the correct preposition?

Thanks.

Tom

If they're in the water, use in. If they're not, use at.

I would say if you know they're in the water, use in. If you don't know, use at.

If you don't know if they're in the water, I wouldn't say "swimming".

They're at the pool. (We don't know if they're swimming.)
They're in the pool. (We know they're in the water but we don't know what they're doing.)
They're swimming in the pool. (We know they're in the water and we know what they're doing.)