The difficulty in regulating mining in international waters are precisely why companies are rushing into this market. It's much harder to stop something that's already been started, and regulatory agencies are notoriously slow.

What we do know of seabed mining is that it's incredibly destructive to marine ecosystems. As Peter Watts writes,

Very little research has been done on the environmental impacts of deep-sea mining. The only real study was undertaken thirty years ago, led by a dude called Hjalmar Thielon. It was a pretty simple experiment. They basically dragged a giant rake across 2.5 km2 of seabed, a physical disturbance which— while devastating enough— was certainly less disruptive than commercial mining operations are likely to be. Today, thirty years later, the seabed still hasn’t recovered.

But what's more concerning is what we don't know, as very little research has been conducted on its impact. Moreover, many of these ecosystems are largely uncharted. We could very well destroy something before we have the chance to understand it.

On a higher level, this is what happens when you attempt to solve for one variable (climate change, in this case the transition to renewables and its associated mineral demand) instead of looking at an issue holistically (i.e. the total integrity of our biosphere).