Yes, you can swim on your period
Breaststroke, freestyle, doggy paddle, or lying very still on a pool noodle. The lining is sewn in, so there's nothing to insert and nothing to change in a wet cubicle.
How much do they hold
Around two teaspoons, roughly one tampon's worth. Light and moderate days, or backup alongside a cup.
Will it leak in the water
Not on a light or moderate day. Around a tampon's worth of capacity, and it keeps working when you get out, which is when most people actually worry.
What about a heavy day
Swim earlier in the session or wear a cup underneath. This is built for light to moderate, and it's better to know that on the sand than in the water.
Can anyone tell
No. They're cut like swimwear because they are swimwear. Nobody at the pool is doing forensics.
How do I wash them
Cold rinse straight out of the water, chlorine and salt both. Cold machine wash, no softener, line dry out of the sun.