> money being extracted from local economies
> the coffers of the ultra-rich
For someone complaining about "environmental debt", a term I have never seen before, you do use a lot of tired phrases.
I also don't quite believe that international shipping is quite as amendable to scrappy little startups with little money as you make it out to be?
> other than ostensibly environmental protection
Yes, the benefit of the proposed scheme is environmental protection, and therefore it is no surprise that you identify it as it's primary benefit ("ostensibly" being a meaningless qualifier used to vaguely dismiss the idea without feeling the need to argue the point).
Both the principle as well as the mechanisms are well established. Car insurance comes to mind, or really any liability insurance.
An insurance scheme would, in fact, significantly lower the capital requirements compared to straight-up bonds, in the same way that you don't have to put up a million $ to get that amount of coverage for your car.
> actual implementation in practice
Require insurance against environmental damage caused by any vessel. Deny entry to foreign vessels not complying.