And a safety boat with somebody with a radio to reach out to passing ships. This is on a river, so if the diver is in the channel the ships can at least give the diver a slow bell.

it's real hard to judge the actual depth from the bottom looking up, he might be 20 feet down or 50; depending on water clarity, how sunny the day is, etc. it could look exactly the same from that perspective

every largish commercial vessel will have a fathometer that tells them how much water is below, and you'd be surprised how skinny some freighters can run when they aren't loaded down. the St. Clair river is a major waterway that does a ton of commercial ship traffic so it's probably just a captain that knows the route really well.

probably a fun place to dive because it looks like there is a lot of interesting junk from all the freight traffic over the years. but no way would I do it without multiple indicators on the surface including a manned boat

Ports I've worked at have required ships to have a minimum of two feet of under-keel clearance in the channels. If the boat cleared the bottom, then the water is deep enough.

Dang.

Seems like the kind of place where diving is illegal or done at your own risk kind of thing, like skateboarding on a highway

I reiterate

Ahhhhhhhhhhhhhhhhhhhhhh