One of our ponds is designed to be two-thirds swimming pond, one-third fish. The lower two-thirds has no cover or structure. We have a pier, made of welded channel iron on a 2 7/8" drill stem frame, with cedar planks screwed into into it with brass screws. We chose cedar because it's plentiful and we have a sawmill nearby. The dock is a focal point for swimming. Kids gather, shove each other into the water, or jump. It's a cool place, and will have a diving board and ladder before long.
The princess Debbie designed a zip line for the kids. It's attached to a custom designed steel pole buried in concrete 36" deep, held in place by guywires at each end.
We have a small beach, make of finely ground granite from a nearby abandoned railroad right of way. The beach has a small retaining wall in water three feet deep, with shadecloth underneath, with the crushed granite on top of the cloth.
Off the dock, the water is 14 feet deep, and we have a waterfall in the upper end of the pond, coming from a pipe which is fed by the well. We try to keep the pond level full with the well, especially during summer.
Debbie and the kids, their friends (and I)enjoy swimming in the pond.
By the way, we also irrigate the lawn from the same pond.
The only caution about swimming is to pay attention to your water quality. Don't let it become stagnant. Aerate, if needed.

Teach a man to grow fish...
He can teach to catch fish...