With an automobile on public roads, you need a drivers license. In CT, legally blind will prevent you from getting or keeping a drivers license. I think most states are the same, but I'm not certain about MA.
As for manifestly unsafe, I must pass or see half a dozen vehicles a day on the road that I consider 'manifestly unsafe' - no tail lights, bald tires, obvious wheel shimmy, sheet plastic windows, etc. No one pulls them off the road.
With boating, it's a different story. Seems to be a) jump in, b) hit the throttle, c) go. Yes, there are state boating safety courses and certificates, but no coordinated set of requirements for operating a vessel. Has anyone ever heard of anyone being arrested for operating a boat without a boating certificate?
So, back to 'blind guy', what statute is he violating? Perhaps 'failure to maintain a proper lookout', but they've turned a 'blind eye' to this rule regarding the single handed racers.
Perhaps the future will see the USCG preventing single-handed racers from departing on 'manifestly unsafe voyages'. Until such time, they should not have responded unless to a distress call from the ship. IMHO, their reaction was unwarranted and unnecessary - and as a taxpayer, I resent being sent the bill for it.