Fill a large pot with about 2 gallons ocean water or heavily salted water (should taste like seawater).
Layer half the rockweed seaweed in the bottom of the pot.
Add potatoes on top of seaweed, cover with lid, and bring to a boil. Simmer for 5 minutes.
Add halved onion, lobsters, and corn on top of potatoes. Cover and simmer for 15 minutes.
Add clams during the last 5 minutes of cooking time.
Steam until lobsters are bright red and clams have opened completely.
Remove all contents from pot and arrange on serving platters.
Serve immediately with melted butter and fresh lemon wedges.