Your daughter is only having a very natural reaction. Of course as long as she is wearing tampons she can go into the water and dive and noone will ever know.
But it does take a while for any girl to get used to having her period. Can you imagine shedding blood from the most intimate part of your body ? A part that a young girl might view as "dirty" as it is related to sexuality ? It's a weird feeling that I wouldn't be able to explain. A mix of shame, selfconsciousness, fear.
Besides, it is something very mysterious, as every woman is different and needs time to know how her specific cycle works (different flows on different days mean using different "equipment" to make sure you stay dry).
In my opinion, the best thing you can do right now is to let her get used to becoming a woman and be understanding if she doesn't want to go diving on those days.
Depending on how open of a relationship you have, you might talk with her about it, suggesting that you schedule your diving trips when she doesn't have her period until she feels more comfortable (keep in mind though that at the beginning periods are often not regular).
But if you sense that she doesn't like to talk about it with you, just let it go, don't question her or try to have her talk about it with other people you feel might "help" (she is not suffering from a problem and probably has lots of girlfriends whom she can share that experience with) and be patient. She'll eventually grow out of that selfconsciousness, but that may take months or years.
It's just a normal part of growing up and becoming a woman and there's not much you can do about it.
Besides, a period only lasts one week a month at the most... That still leaves you plenty of time to enjoy diving as a family.
Good luck
Anne-Laure
