It's the stuff of nightmares – you go in for a quick swim at the beach, and walk out with bites all over your ankles that won't stop bleeding.

But that's exactly what happened to 16-year-old Sam Kanizay on Saturday evening after a swim at a Melbourne beach.

He was feeling a bit sore after a football game, so went to ice his legs in the cold ocean water.

"When he got out, he described having sand on his legs, so he went back in the water," his dad Jarrod Kanizay told AAP.

"He went back to his shoes and what he found was blood on his legs… They ate through Sam's skin and made it bleed profusely."