[Rin pulls a face. How is it even possible to swim this badly when you were born in the water? Rin just doesn't get it. Rei's seriously cursed or something.]

Man, your tail is going all over the place. Maybe you should focus on moving that normally first. Just concentrate on fluid strokes. Like this.

[Rin leans back to demonstrate smooth movements of sharper finned tail.]

You'll find it easier to keep from flipping to the side once you've got that down.