She doesn't need to go that fast, they could probably fly back to the city like we always do.
The ground issue doesn't really matter in my opinion, we can fight with fire-type pokemon literally underwater. And well it is true that Sirius could do those things, but why didn't he do them to the main character then?
And well even the wild Whiscash swimming in the lake could have defeated Sirius and his grunts, I don't think it would have been impossible for her (what if the meteor grunts were actually killed by the Whiscash lol).
But all these considerations are beyond my point, what I wanted to say is that she had other options but she (as Titania did later) chose the "easy way" so that she could forget her opponents were human beings.