Kelp is rich in iodine, something many people don't get enough of.
Instead we ingest large quantities of nitrates and perchlorates, which can cause thyroid problems, especially in people with an iodine deficiency. The nitrates mostly come from preservatives in meat. The perchlorates mostly come from solid-fuel rockets and runoff from military installations, and are concentrated in the leaves of irrigated salad crops.
To learn more about these problems, here are some articles from google scholar.
Kelp has a high iodine content. The iodine content varies greatly from one sample to the next.
Excessive iodine can aggravate thyroid problems. Tell your doctor what you're doing and don't over do it.
Step 1: Gather Kelp
Only gather kelp that's either drifting freely or has washed up on the beach. If there's current, here's how to tell if kelp is anchored without even touching it: If the "head" is pointing upstream, the kelp is anchored. If the "head" is pointing downstream, it's drifting freely.
If you still see a lot of the stem floating at high tide, the kelp is drifting. At low tide you'll see a lot of stem even on anchored kelp.
Here I am off the coast of Northern California. I found this bunch of kelp drifting in the current.
Put it in a bag or other closed container before you get to the beach, you don't want your kelp to get sandy.