"My father and grandfather are swimming today."

Translation: मेरे पिता और दादा आज तैर रहे हैं ।

[pradeepan90]

Won't it be as "tairte hain'?
As swimming here is not verb but noun. So the translation should be of simple present tense I guess.

[VPGixM]

It is present continuous tense given here for transalation so you have to use tair rahe hain.