I agree with you, personally. Few calories. But in the west right now, this is often seen as a positive (due to obesity). It can be a healthy diet/snack food.
Predator fish like Tuna can become heavier in toxins only because they are the top of a food chain gathering and concentrating said toxins.
Seaweed is just a plant. It grows, then dies if not harvested. It is no higher risk than wheat.