A pad absolutely will not work for swimming. It will quickly absorb all the water so it won’t be able to absorb your blood, it will swell up to an embarrassing and obvious size, it won’t stay snug in your swimming suit and may slip out and float to the surface. Tampons and menstrual cups collect the menstrual fluid before it even leaves your body, so the chances of leaking are pretty low.  You can wear tampons for up to 8 hours and cups for up to 12, so you can go from sunning to swimming  to playing beach volleyball without having to dash to the bathroom. Look for tampons labeled “active” or that are designed to be worn while playing sports. These are less likely to leak and are designed to stay put while you’re swimming, running, or lunging to catch a frisbee.  If you’re worried about your tampon string showing, just bring a pair of nail clippers and carefully shorten the string after you insert the tampon. Otherwise, just tuck it into the lining of your suit and you should be fine. When you enter the water, your period can stop or become very light. The pressure of the water can act like a plug or little airlock and keep the menstrual fluid inside. But this is not guaranteed to happen, and you shouldn’t count on it. Put some back-up tampons in a little bag and then stash them in your beach bag so you’re not caught without. Your flow could be heavier than expected and you might need to change your tampon a few times, or you might stay at the beach longer than expected and run up against the 8-hour time limit for wearing a tampon safely.  Keeping extra on-hand will give you peace of mind, so you can relax and enjoy yourself instead of wondering where you’re going to find a fresh tampon. Bringing extra tampons might end up staving the day if your friend unexpectedly gets her period or forgot to bring her own supplies. This is not the time to wear your white bathing suit. There’s always a slight chance of leakage, and since you won’t be wearing a panty-liner to protect your suit from leaks, choose a suit in a...

TL;DR in Chinese:
如果你打算游泳，月经杯或者卫生棉条是必不可少的。 带上足量的经期用品。 穿深色的泳衣。 为了应对痛经，带上非处方止痛药。 如果你打算去度假，试试将月经延后，或者直接跳过本次月经。