Want to know what possibility for harvesting a particular element is, from the ocean?

click on it in https://www.mbari.org/know-your-ocean/periodic-table-of-elements-in-the-ocean/

( scroll down a bit )

& see a new page open up, identifying how abundant/not it is, & the top-to-bottom-gradient of it..

Rare-earths may be sufficiently harvestable from the ocean using Metal-Organic-Frameworks, for coastal de-salination plants, or tidal-power-plants, if done at scale..

( which would break national-monopolies, wouldn't it? )

just don't bother trying to mine iridium through that method: you'll never get enough to make it worthwhile.