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I've never used one, but I've heard really good things about them. So far everywhere I've been diving, at that time of the month, I've had the luxury of having a bathroom handy. If I ever get to go on a boat though, and I'm going to be in a similar situation such as yours, I will be purchasing one of these!
And everything I've read says that sharks are NOT attracted to menstrual blood. Don't worry about it!
Rhonda