There are countless carbon sinks within the ocean. It finds its way into the shells of creatures (calcium carbonate) and hangs around for a very long time in solid form. And lots of creatures die/defecate, that sinks to the bottom of the sea, and much of the carbon there doesn't rise back up since not all of it is consumed.

And when you're spreading seaweed over a fish farm, a good chunk of that is flowing back out into the ocean and contributing to the cycle of carbon deposits.

https://animatingcarbon.earth/fish-the-excretion-effect-boos...