That’s one part of the question. The bigger more complete question is ‘the market’, because you need more than one customer.
Essentially this is an agricultural business model, even though it’s a wild harvest. By this I mean you have a product that is seasonal (at least it grows more during certain seasons, one should also look into seasonal limitations on the harvesting license and weather patterns that would affect harvesting operations), spoilable, and meant for food/fodder. So the agriculture model fits as well as anything will.
So how many customers are there in the potential market, how many existing and how many potential? Is the market saturated with producers or is there an unsatisfied demand? What is the availability of product- are the existing harvesters taking all of it? Can the amount of kelp growing wild be improved by judicious “cultural practices” at harvest?
Are there grades of kelp that can be sold at a premium? Is there a fall back “commodity” market for excess? What about value-added kelp products (pelletized, bundled, baled, chopped, dyed, ground, sheets)? Can value-added processes be applied asynchronously (e.g. with excess product stored until a period of low demand or during bad weather) or do they require fresh product?
Does shipping cost factor into limiting your market? What are the communication channels for reaching customers? Is it a vertically integrated market or do you use channel partners?
These are some of the questions that would go into creating a business plan for a kelp harvesting business, I would think.
Look for business plan templates aimed directly at this, but don’t limit yourself. Look also for business plans aimed at wild harvest of other items: mushrooms, truffles, ginseng etc. It might give you some ideas on organization and marketing.