Where do marine creatures store carbon in their bodies? What form is it in?

The tissues other than blood contain most of the CO2 stored in the body. Over 90 % of this tissue CO2 is stored in bone as carbonate (65, 155).

When they die, their carcasses sink to the seafloor, bringing a lifetime of trapped carbon with them. This is called Deadfall Carbon. On the deep seafloor, it eventually can be buried in sediments and potentially locked away from the atmosphere for millions of years.