i once went to island on my second day of period (the day where my period is the heaviest)
i didnt wear tampon, just the usual pad.
and i did went down the sea -- swimming, playing water games, snorkelling etc etc...
but it didnt leak, in fact the blood didnt come out as much...
friends said it was due to the water pressure...

months later, i went to another island trip, and coincidently i was period again -- and it also didnt leak out....

so, from my experience, i can still go under water even when i am on period, but then again...different person different body (",)