The 50-year-old Congress MP swam for 10 minutes before returning to the shore.
Boxer Vijender Singh shared a photo of Rahul Gandhi commenting on the Congress MP's "abs" during his visit to Kerala, where assembly election will be held in April.
Mr Gandhi had dived into the sea with fishermen as part of his campaign to support their issues.
In the photo shared by Mr Singh, the Congress leader is standing on a boat, his clothes wet from swimming. It was shared widely on social media.
"Abs of a boxer. Most daring young fit & people's leader. Way to go @RahulGandhi Ji," Mr Singh tweeted.
Some others also tweeted:
The 50-year-old Congress MP swam for 10 minutes before returning to the shore. He was accompanied by four Congress leaders including KC Venugopal and TN Prathapan. He also tried his hand at fishing with them.
The fishermen served Mr Gandhi bread and fresh fish curry that they prepared on the boat.