I was just going to direct you to the Achievements Thread (since they are speculating there), but I just noticed it's known now. Last post:
"So I see someone else finaly got the achievement, so I will confirm it for all of you. It is for swimming, and don't worry it is not 1kk steps like the other ones, it is just a simple around 100 000 times, took me 7 hours to get it.
2 points, 1 star and the description is:
One thing's for sure: You definitely love swimming. Hanging out on the beach with your friends, having ice cream and playing beach ball is splashingly good fun!"