At least for the US, any spy or saboteur who wants to enter can do so on a student visa, I am sure. Just have to pony up for tuition at any of a number of schools who are desperately looking for students who will pay.
There is more time for a background check that way. There are many ways to work around the difficult US entry requirements if an attacker is determined, but if something like this bypasses the process attackers will go for that: boats are cheap.
It's not clear to me that an ocean-going fishing boat would be cheaper than college tuition. Is it? Would love to see some data. https://horizonship.com/ship-category/commercial-fishing-ves... has prices a good bit about even 4-year tuition for all the things that list actual prices.