Paul Franklin, from Aliso Viejo in Orange County, fell and grazed his knee on rocks while on the family trip.
The youngster's mother, Rachael, was squeezing the sore on his knee three weeks later when a sea snail burst out, 'New York Daily News' reported.
"His knee was size of orange and hot to the touch and he's limping," she told Sky News.
She said the snail "looked like a rock - it was a black thing and I put it on paper towel and it had whirls and I turned it over and it was a sea snail, and I just like had to laugh."