Can I use "in" instead of "at?" How would it be different?
Answer from a Native speaker
You could, but it would be a little different! "In the swimming pool" would mean that they are physically inside the swimming pool, whereas "at the swimming pool" means the general location of the pool and not the specifically inside the pool itself. Ex: I'm at the mall. => at the location of or in the mall. = I'm in the mall. => inside the building itself Ex: I'm at the restaurant, are you coming? => at the location of or in the restaurant, possibly parked outside. Ex: I'm in the restaurant, and I got us a table. = I'm at the restaurant, and I got us a table. => here "at" also means "in"