I'd say no to a coot. They don't dive that much just for the heck of it and they are rarely ever alone. Plus there shouldn't be any imature coot around this time of year. Also coot don't lay their head back like this bird, they always have their heads forward or straight up.
It is a pied-billed grebe. We see a couple of these every year while hunting. Usually is two of them together but sometimes only one. Theyjust swim and dive around all morning while you are shooting. Nothin bothers them to much. They are illegal to shoot in WI.WCL