The short answer is yes, if the rules are followed, it is technically legal for personal size sailboats, 16ft (4.9m) or under, to be out in a thunderstorm in Nantucket Sound. The Sunfish, which is 13.9ft, (4.23m), falls into that category. However, it's not smart, and is highly discouraged!
Massachusetts State Boating Laws break watercraft down into two classifications; those with motors and those without.
For use in public waterways, registration is only required for watercraft which have a motor, even if the motor is not the primary means of propulsion. Watercraft without motors do not need to be registered with the State. The Sunfish was designed without a motor, and I believe that's still true. Therefore registration is not necessary, and would not have been the cause of your dad's trouble.
Federal Safety Regulations:
All boats, of all sizes, registered or not, must comply with Federal Regulations for safety. Federal laws are classified by boat size. The smallest category is 16ft (4.9m) or smaller. As I said above, the Sunfish is 13.9 ft, (4.23m) so it falls into that category.
Among other things, boats in this category are required to carry:
- Life Jackets (PFDs):
One Type I, II, III, IV, or Hybrid Type V for each person. Hybrid Type V must be worn at all times to meet Coast Guard regulations.
- At least one oar or paddle
- Whistle, Bell or Horn:
Any device capable of making an "efficient sound signal." This regulation is applicable for boats up to 65.6 ft. (20 meters). Larger boats have stricter sound requirements.
The Massachusetts Marine Trades Association Massachusetts Boat Safety Tips website clearly lays out the rules for all varieties of boat. All boaters would benefit greatly from reading that site and its related links. They have a list of potentially life-saving safety tips, two of which may have been especially pertinent in the case of your father's incident.
Be weather wise. Sudden wind shifts, fog, lightning flashes and choppy water all can mean a storm is brewing. Bring a portable radio to regularly check weather reports.
Tell someone where you're going, who is with you, and how long you'll be away.
If your dad complied with legal rules but not safety rules, that would have been enough to get him in trouble. Putting other boats and boaters at risk of injury just to have the adventure of sailing in a storm is a bad idea. Your suspicion that he might have been considered reckless makes sense.
Also, it's disrespectful to use precious agency resources when they have more important problems to attend to during a weather event. Impounding the boat and punishing the sailor would have been appropriate.
The waters surrounding Nantucket are governed by a complex combination of Federal and State jurisdiction, mostly for environmental protection reasons. A few different agencies monitor watercraft behavior.
I've had some trouble locating enforcement measures for small sail boats in current times. Some sites say that agencies are frustrated when it comes to enforcement, mostly because revocation of license is the first line of defense, and small sail boats don't need to be licensed. Some infractions are just overlooked, unless they're very dangerous, or committed by repeat offenders. Even with the rules in place, impounding doesn't seem to be used as it was so many years ago.
Nantucket's population has had tremendous growth in the last 50 years. The island was much less populated at the time of the incident you described, so local enforcement was probably more personally involved. The fine itself was probably for the infraction, the cost for the boat to be towed to and stored in the impound area, or both.