Does kelp sink when it dies?

Kelp sinks only when small sacks of air attached to the plant are broken. If the sacks are broken while it is alive, it will sink and die. If the kelp dies, but the air sacks are not broken, it will float until enough are broken to make the kelp too heavy for the amount of air in the sacks. However if it is dead for a long time, sometimes weeks or months, it will decay enough for the sacks to rupture. The bottom line is that kelp rarely sinks soon after it dies, it will most often end up on shore.