"You are swimming today."
Translation:आप आज तैर रहे हैं ।
हो only agrees with second person singular and plural, तुम(you) and तुम लोग(you/you all). हैं agrees with हम and with third person plural like with वे, ये etc.
See tips by the mods:
The verb रहना agrees with second person singular/plural in the form of रहे.
The structure is root verb + correct conjugation of रहना+ correct conjugation of होना
See tips on the conjugation of रहना
Yes , if you want to write "तुम आज तैर रहे हो।" or "तुम आज तैर रही हो।" in translation for "You are swimming today" ,then, both are correct and should be accepted.