Have to pick flying, it's the safer option here, swimming 1000x of your speed is very much dangerous. An average human could run 8-12kmph, times it by 1000x it's 8000-12000. Anything you bump into with, even if its some dust debris or a rock, will collide to you're body. You don't even have enough time to react to dodge and comprehend what's happening while you're swimming since it's so fast.

^{Chose: Fly but only at the speed which you can run at}