It depends where you are. It probably looks like a person swimming. It could be different though.
A crossed diamond is used to mark a swimming area. This is considered a 'Boat Exclusion Area," and may be placed outside spots such as a dam, rapids, swim area, etc.
White tubular buoy, orange diamond with black cross in it. Tells boaters to keep out.
a human figure in profile in water (wavy line) from the torso up, with an arm raised with elbow bent (like an Australian crawl stroke).
This is a non-lateral marker. It can indicate a controlled area, such as no wake. It can be an informational marker, a "keep out" marker, or a warning marker (Dam, Rock, etc)
keep out of that area
A circle marker is a symbol on a regulatory marker that is used to mark a no wake or idle speed area.
keep out of that area
I've been to the beach and these are the markers you'll usually see farther out in the body of water. Think of this marker as a "No Swimming beyond this point" sign. These markers are usually set out in the deeper water. I don't know the exact measurement of where they'd be placed but my guess would be they'd set it out on the edge of where it just gets deeper than 5 ft. No one is allowed outside those markers and no boats are allowed inside those markers. These markers show the public of where it's too deep for them to swim safely and these markers show the boats where the swimming zone is.
Controlled area
Mile marker #123.
A designated swimming area, for one.