Can you have the Beach screensaver on macOS Big Sur?

Sort of. If you go to this thread from 2015 there is a Dropbox link to the original beach.slidesaver file. Somewhat amazingly after all these years, it's still a live link.

I don't believe the actual .slidesaver file will work on Big Sur, but you can download it, right-click on it, and choose Show Package Contents. Inside Contents is a folder called Resources, and inside that are the .jpg images that are displayed when the screensaver runs.

Create a new folder in your Pictures folder, call it "Beach" or similar, and drag or copy the .jpg files from ~/Downloads/beach.Slidesaver/Contents/Resources/ into ~/Pictures/Beach/

Launch System Preferences, click Desktop & Screensaver. Choose the Ken Burns screensaver. Click on the Source pull-down, the Choose Folder and navigate to ~/Pictures/Beach . Click on the Beach folder and then the Choose button.

You'll now have the images from the original Beach screensaver in a Big Sur Ken Burns screensaver. But they're really old images, and the quality is low for large modern displays.