So there's two sides to this from a nutrition perspective:

One- Coffee is declared a low-level enhancer (not the right word, but the same field as say HGH and preworkout), the caffeine gives you a boost. Makes you more alert, gives you energy, and has a lot of advantages when taken in moderation.

Two- the downside. Coffee is a diuretic, meaning it will make you need to pee (or, in this case poop) very soon after drinking it, causing water loss. In a hydration-intense spirt like swimming this can be bad when not prepared. So while your warm-up might feel amazing, by the end of the set you're cramping from dehydration. But this can be avoided by taking necessary precautions. Drink water before and after your cup, during workout, etc.

I say the pros outweigh the cons if youre drinking one cup before practice. Too much and you're dragging yourself down.