Spanish is more dominant than English in Puerto Rico; driving far from the airport you can get a feel for what it's like to be illiterate if you can't read Spanish. Is she very fluent?
The business venture sounds doomed. More an issue is the prospect of throwing away funding for a good formal education, and a marriage that, if her other decisions shared here are a good indicator, may not be wise. Sad.
I'd emphasize that it's unlikely that they can show up without advance training, skills & qualifications, and no starting capital, and establish a profitable business in a highly competitive market for a narrow profit margin industry where the locals are probably already well-established, will be favored over immigrants (yes, I know Puerto Rico is a U.S. holding), and where they likely have no roots (maybe he does?) or contacts.
Also, it's likely they'll go heavily into debt, be unable to afford health insurance, and be unable to provide well for their children (sometimes maternal protectiveness, even in advance, might help?).
You can even point out they need the knowledge & skills to get into the business. She should pursue Dive Master, then Instructor, here in the U.S., and both should do some business classes in college to get the skills/knowledge needed to run that business, and a convincing qualifications base to get a bank to load them the start-up capital. She could work on an online MBA program or some such.
If you can send them off on that path (which would make better sense even if they do go through with it successfully, which I doubt), it should delay them a few years (even with 'zero to hero' instructor training options), and reality might seep in and wake them up in the interim.
There's only so much you can do to save people from themselves. Ask any parent of an alcoholic or drug addict. It can be like watching a slow motion train wreck, but sometimes, watching & praying are about all you can do.
Richard.