Non-U.S. citizens should not spend more than 179 days per calendar year in the U.S. in total
If you spend 180 days or more, the U.S. considers you a resident for tax purposes and the U.S. is the only country in the world, other than Venezuela, that taxes residents and citizens on their worldwide income. This means that if you spend 180 days or more on any given calendar year, the U.S. expects you to "file" (declare) taxes on all income from all countries for that year. Being considered a U.S. resident for tax purposes has no benefits of any type. It just means that you pay taxes as if you were a U.S. person, even though you are not.
The amount of time and/or re-entries that a visitor gets is nearly arbitrary. Across about 30 years of experience (personal, and colleagues and friends) I have seen people get:
a) Longer than the validity or overhang of the visa (this was error by the government employee and if the user had made use of it the adverse consequences would have been massive for the victim, not for the employee). Seen this more than once
b) The full six months
c) Three months or less
d) The exact number of days that the person indicated was visiting for
Example: you come for a meeting, three days, you get six months. You leave on the fourth day. You come back in a month, you get three months. You leave after three days. You come back in two months, you get six months. Does it sound random? It generally is.
If I were visiting in a boat, I would plan a visit six months or shorter.
It is possible to apply for an extension of visit, which may be granted, but the migration employees are not the tax employees who will bite after 179 days per calendar year.
The above is related to "persons".
Related to "boat", boats from about 25 countries can obtain a 1-year cruising license, renewable by the boat briefly leaving the U.S. Our boat is now on its third cruising license (third year). Countries not on the list cannot get a cruising license, and it becomes very difficult to do any type of coastal cruising in the U.S. without a cruising license.
At anchor, St. Marys, Georgia