A "hey, be aware of this craft" might be in order
I am not experienced with U.S. coast guard protocol; however I think in the same situation I might have noted the craft name (and registration numbers) and given a heads up to the coast guard or marina personnel as an FYI to them indicating their general direction and possible destination. I would be very surprised if they would not be appreciative, as it could save them time if a mayday went out. In my hiking experience I have run across similar situations of ill equipped and ill prepared people doing things they had no idea could potentially put them in grave danger. I have always made an effort to be sure "somebody" knew they were out there. In at least 1 instance I am aware of, this action saved a life, or at least a potentially frozen limb. I would hope someone would do the same for me.