Nothing special there...we've had a beach at this pond for years....the issue is that the sides are so steep the sand washes out and down to the bottom (16+ feet under water) fairly quickly. My solution up to now has just been to add more sand periodically. But seeing as how I had it drained anyway, I added a retaining wall to hopefully slow the loss.
This also allowed us to move the beach closer to the gazebo.

Just a couple of courses of block, rebar pounded into the bottom through the block, then concrete filling the holes.
The main point here is to make sure that wall is down far enough under water that it's of no danger. The top of that retaining wall will be at about 7 feet down and 15+ feet out. No way for anyone other than me scuba diving to get to it accidentally.