Civil Eats has published an article about seaweed aquaculture in Maine.
There are many varieties of seaweeds, and methods of farming differ, but sugar kelp production works well in Maine. Kelp farming generally involves gathering source tissue from wild kelp and then using the spores from that tissue in a lab to grow kelp seedlings or “sporophytes” on strings that are then attached to ropes strung out just below the ocean’s surface. Leaf-like blades then grow downward while clinging to the rope from what is called a holdfast, hanging below the surface of the water in rows.
Looks like Rhum is for lease
http://www.loopnet.com/Listing/98-Cross-St-Portland-ME/10873918/
http://images4.loopnet.com/d2/Pa-pkBC_leznDVKTTavPE0SnIpuFi4cx6lFAXMN3Xws/document.pdf