How often do sharks attack people at a regular beach area in typical waist high water?

I did a quick search and couldn’t find this specific so I figured I’d ask.

I’m in the northeast US.

Just trying to paint a picture of what I understand to be a typical day at the beach. You’re swimming in the regular designated area with a lifeguard present. You go waist deep or perhaps slightly deeper.

How often have sharks attacked people in such an environment?