How is it enforced, and what are the penalties?
It's enforced by fishing in the high artic being pretty conspicuous. Regarding penalties: With ratification, such treaties become law. There are no penalties specified, but courts are empowered to enforce them by whatever means they deem effective. Environmental groups, for example, could sue the US, in US courts. Beyond that, international cooperation doesn't rely on penalties or similar sanction mechanisms as much…
Nor did the US pull out of the Iran deal. Congress attached a rider to the ratification requiring the president to periodically certify Iran's commitment to the deal to continue with the unwinding of sanctions. Trump declined to so certify Iran, but that doesn't affect the treaty in any legal sense. (It certainly does affect the foreign politics of the situation, but at this point it seems clear that the world knows to ignore Trump and just flatter him into thinking he's important and useful).