Swimming during your period can be tricky. Luna Undies aren’t ideal for swimming because they're worn outside of your body, and will absorb any water they come into contact with - leaving limited room to absorb your period.
Plus, since washing in water is how you get them clean, any menstrual flow already in your undies could seep out into the pool when you get in.
Some people find the blood temporarily stops flowing while swimming because the water pressure holds it in, but only to a point. Once you get out of the water, all bets are off. It *will* start flowing again.
We recommend The DivaCup for swimming - it's internally worn and is an easy solution for an eco-friendly day at the beach.