With amazon fundign research into seaweed farms slung between Offshore wind turbines, interest is increasing in different sequestration routes using seaweeds.
https://www.theguardian.com/environment/2023/mar/01/specieswatch-sugar-kelp-north-sea-carbon-climate-crisis

Verification that sunk seaweed stays down and the carbon locked up is a little harder: https://www.technologyreview.com/2021/09/19/1035889/kelp-carbon-removal-seaweed-sinking-climate-change/

It may be that it's not reliable enough to be worth doing at scale, or it may be that seaweed biomass is more useful as feedstock for novel processes.

This market includes all marine grown biomass that is intentionally sequestered, or the portion that is processed with carbon being sequestered (eg turned to a biodiesal, and pumped into old oilfield). The 100gt target is net, and fuel emissions, fertiliser input and other emissions created by the seaweed farming must be compensated for in addtion to the 100gt.