Good subject, but one important error in your post. The Cabotage regulations are different for St Thomas and San Juan - as they are US Territories. On a typical San Juan cruise, if you missed boarding in San Juan, you could board in St Thomas with no penalty.
As a Cruise Ship Hotel Manager I have been dealing with the PVSA - and it's bizarre and spotty enforcement by US Officials - for about 35 years.
Yes, if a passenger misses boarding the ship in a US port and tries to join later in the cruise, he is usually guilty of violating the PVSA. But we always let him board, cautioning him that there MAY be a $300 penalty. We report it to the Port Agent, the Port Agent reports it to the Coast Guard and US Customs, and we wait to see what they want to do about it.
The same thing happens if a passenger wants to leave the ship in a US Port before the end of the cruise. He has violated the PVSA and will most likely be fined.
The same thing happens if you die before the end of the cruise. If the cruise started in a US Port and your corpse leaves the ship (before the end of the cruise) in another US Port, your estate will most likely be fined $300 by US Customs.
Why does the PVSA continue to haunt us? It is an outdated law that seems to no longer have any purpose.
Not quite true.
The PVSA and associated regulations require that nearly all goods imported to Hawaii - and nearly everything, including pineapples, is imported in Hawaii - must be carried by US Flag cargo ships. This dramatically increases the cost of living in Hawaii, and keeps the last two remaining US Flag Cargo companies in business. It also provides jobs for several thousand US Merchant Mariners who would never be hired by foreign cargo companies.
If the PVSA was cancelled, these two cargo companies would be bankrupt next week - and Hawaii would be a far more affordable place to live.
Five major US Flag Airlines are bleeding money in nearly every sector - except Hawaii. Why? Because the PVSA gives them a monopoly over this market, allows them to price fix, and make big profits there.
If the PVSA was cancelled, Air Canada and Japan Airlines would make flying there far more affordable - and the 5 major US Airlines would be bankrupt and their employees out of jobs.
These conditions also apply - in a more limited fashion - to American Samoa, Guam, Puerto Rico, and the US Virgin Islands.
The PVSA will be around for a very long time.
And yes, there is a way around it. If you embark a cruise in any US Port and leave the ship in a foreign port before the cruise ends in a US Port, there is no PVSA Violation.