Are you up to swimming? Are you fit/well enough to swim?
Are you up for swimming / Are you up for a swim?
Do you want to swim?
Are you up to swim doesn't exist as such - I assume you meant one of the above. Don't worry though - there are many phrases in English that use subtly different combinations of prepositions to give different meanings.
Something even more subtle: Has anyone else noticed, for example, that situp (emphasis on sit) is a noun - a type of exercise - whereas sit up! (emphasis on both sit and up) is a command urging somebody to alter their posture? Two completely different meanings, with only the slightest of audiable differences - if you're a native speaker, try saying one after the other. Even I got confused!