It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

Pretty sure I was underwater for more than a minute
I was underwater for more than 5 minutes and it didn't unlock

UPD: earlier verison of the game has this achievemnt working well.

Devs,please fix it in newer version. or just revert some changes which broke the achievement