> One question I have is how sequestered the undersea carbon really is
Depends on how you interpret "sequestered". In the sense that it's locked away somewhere for a long time, like organic matter frozen into tundra, it's not very sequestered. But as long as there's lots of seaweed there'll be a lot of carbon taken up. Even though individual plants may die and release their carbon on a relatively short timeline, new growth will take up carbon.
Depends on how you interpret "sequestered". In the sense that it's locked away somewhere for a long time, like organic matter frozen into tundra, it's not very sequestered. But as long as there's lots of seaweed there'll be a lot of carbon taken up. Even though individual plants may die and release their carbon on a relatively short timeline, new growth will take up carbon.