These guys are scuba diving for moorings. Explanation, the mooring buoys are secured to large weights on the bottom (usually 6 to 10 cement blocks attached to a heavy chain), once settled into the mud they are a fixed point and assigned to one boat. But in bad winters (which we did NOT get this year) the thick ice on the river can seize the mooring and chain and drag it off its location. After mild winters like this one the float on the surface can separate from the chain or cable and then the drag on the bottom must be found. That is what these men are doing.
I’ve done that, in my twenties, and I can tell you that it’s not like the movies. In any river there is mucho slit and you cannot avoid stirring it up, so the process of finding the mooring is groping in the dark with your hands. While you are trying to keep your place in the current.
Remember this shot of the Norwell/Marshfield launch area, at high tide?
This is Dead Low tide.
Clear what you are looking at? See the rails from the first picture? Its not the Bay of Fundy, but its interesting.
This is why powerboats should observe the low/no wake rule on all parts of the river.