"Maybe there are some unique minerals or harvestable goods around deep ocean lava vents?"

Indeed, and there's already robots to harvest those metals:

They need to be controlled by humans over long, thin fiber optic cables though. Radio doesn't go very far through sea water unless it's ELF which is too low bandwidth for almost any application other than morse code.

The Chinese government is solving this by building an underwater habitat from which human technicians can control the robots far below, despite the violent surface weather of the south China sea which makes operating out of a boat that loiters overhead unsafe: https://www.bloomberg.com/news/articles/2016-06-07/china-pushes-plan-for-oceanic-space-station-in-south-china-sea