I can definitely check some driftwood the next time I'm at the beach (unfortunately I almost never buy fish due to being too busy/lazy to cook anything other than spaghetti haha) but I doubt the radiation will be detectable higher than background.

I noticed that map in the thumbnail lists radiation levels as Bq/m^3. The red zones were 10 Bq per m^3, so 10 decays per second per ton of water. I could be wrong, but I doubt this is even detectable on this detector - Am-241 sources containing over 30,000 Bq in a tiny point source are somewhat detectable but don't blow out the meter or anything. 10 Bq spread out among an entire cubic meter of water would be less radioactive than potassium salt at the store. I'm sure it would raise your cancer risk slightly over time if you drank it nonstop, but the human body contains enough potassium to easily outstrip this radiation source, unless the Cs-137 is being stored in the body for long periods of time.

I apologize if that was covered in the video but I honestly don't have time to watch it right now - massive amounts of work has piled up this week. I plan to come back and check this out later this week.