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Really? What about when they're ɧoɱosɛҳųαƖs, is it okay for them too?

As a general rule.  If it temps a man to see naked men, then he should avoid the occasion ... obviously.

Can anyone say that going swimming is a necessity of life? It's not like "simply never go swimming" is such a crazy, out-of-this-world suggestion like growing all your own food, producing all your own goods, surviving in the wilderness, etc.

You just simply: never go swimming! It's as easy as never going biking, never going canoeing, never going skydiving, etc.

Your "bucket list" might look like ____ compared to other worldly peoples' lists, but who cares? Catholics should only have ONE mandatory item on their bucket list, which is prioritized before all others:

[ ]  Be found pleasing to God, blameless before Him, and in the State of Grace at the time of death

Everything else is just extra fluff.

If that item isn't checked off at death, it won't matter how many places on earth you've been, sites you've seen, experiences you've had, or anything like that. It will all be for naught if you find yourself in hell for eternity.

I'm sure heaven will be more fun than any swimming experience on earth.

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So, by this standard, we'd be better off hiding in our closet all week and just showing up for Mass on Sunday, and then going home.
Otherwise we'd be a home-aloner, and everyone knows that's no good.
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Then we could check off that item at our death, and go to heaven.
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Never go scuba diving, never get a driver's license, never fly a kite, never play baseball, never learn to ice skate, never jump off the high dive, never ride a ski lift or ski down a pristine mountain of fresh powder, never try windsurfing, never fly a helicopter, never hang glide, never drive a formula 1 race car or a dragster, never fire a high-powered rifle, never run a race, never pole-vault, never watch a movie, never go trick-or-treating, never have a mixed drink, never go to a philharmonic concert, never go bowling, never learn to ride a bicycle, never learn to ride a unicycle, never pop a wheelie on a motorcycle, never learn to ride a motorcycle over basketball-sized rocks while going downhill in the desert, never race motorcycles in a sand wash, never fly a helicopter, never operate a drone, never use a GPS or a cell phone, never use the Internet, never walk a tightrope, never go skeet shooting, never go hunting, never go fishing, never sail a boat, never ride a horse, never rope a steer, never throw a party, never have dessert, never cook breakfast, never tell a joke?
Why?
Well, it might be the occasion of sin!

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I heard of a family who set up an above-ground pool in their back yard so all the children could splash around in privacy. Then one day when the family activities were as usual, very busy, one small child found his way into the pool assisted by an older, retarded sibling, and drowned. They drained the pool the same day and got rid of it for good.

This brings up another good question related to the topic:

It was either Pius IX or X who said mixed-gender swimming was an occasion to sin.  Modernism has infected us all to such a degree that we don’t even realize it...

What about siblings?  My children are young enough yet that this hasn't become a major issue, but it's getting there quickly.  What do other people do with their children?

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So, by this standard, we'd be better off hiding in our closet all week and just showing up for Mass on Sunday, and then going home.

= I like swimming in crowded public beaches. 

I seriously doubt your children are incestuously attracted to one another. Just have them dress modestly regardless, even if there's no one else around it gets them used to it at the very least.

I agree I don’t think it’s likely that children will think in that way but equally we do have very strict rules (absorbed and acted on with no comment) about modesty at home and in public. 
We do allow the children to swim on the beach but beach clothes are strictly for the beach and the water. Shorts and tea shirt for the boys and a dress for our daughter immediately otherwise. 
Boys wear proper loose fitting and longer leg shorts and our daughter a swim dress.