Quite often the visibility at slack water isn't too bad so you should be able to see things.
Light can be an issue as it doesn't penetrate too well and you lose the red end of the spectrum early on.
The other problem with these things is if they are fixed focus or not. Some auto focus ones are pretty confused by particles in the water.
Despite the general view, there is often reasonable vis in our East Coast waters.
I have been known to dive in local waters - especially to retrieve items lost overboard by fellow boat owners but haven't too much problem seeing things - as long as I didn't stir up the bottom.
I've seen one of these in use. It was very impressive, but with one drawback, there is no directional control over what you are looking at. Perhaps a cheap telescopic fishing rod to tie it too would overcome this.
G.