If you are saying that "to practice" (practicar) es fun = es difertido.
But if you are saying "swimming" (la natación is fun) = es divertida.
Or, in other words, "climbing" (to climb) is fun = es divertido
BUT if the mountain (la montaña) is fun = es divertida.
You need to know what the subject is, or what exactly you are comparing.
If the activity (la actividad) is fun = es divertida.
Does that help?
Sra