Who knows, probably. Your relationship with the OSV company is at will employment. If you don’t sign the agreement to not join a union you’ll of rather “understanding” to not join a union they’ll tell you to gtfo the office.
There was another mention of this in another thread but, it said 5 years.
It could be that now, I can only relate to when it mattered to me. The rules may differ between the MEBA and MM&P.
The way I see it, most of the non union fleet rides on the lobbying coat tails of workers unions. Having worked for an oil field company, I can assure you no one pays anything to lobby for US mariners or the Jones act. Not sure how much CAMM does for advocating compared to the unions. Still have not found any organization that takes donations to lobby for US mariners.