You could absolutely write a law that prohibits operating, advertising, selling, or having their point of embarkation inside country X for diving tours on non-certified vessels. Or something along these lines. I'm sure a lawyer could come up with something better.

Making the commercial part illigal in many countries wil make it definitely more clear that you put your own life at risk. If people with that knowledge still want to (likely )kill themselves that's not a problem. The budget for rescue missions wil also be more reasonable for in most countries explicitly unsafe subs.