We got a truckload of seaweed from the ocean cleanup. It was free but there were a lot of conditions required before they would give it away. They wanted to make sure it would not end up back in the ocean. This debris is regularly dragged out of the ocean and contains fishing nets, plastic and other marine debris. Most of the seaweed were brought here attached to the hulls of ships and are not the kind of seaweed we want around here.
It was not washed and a lot of it was added to the compost pile. The pile took over a year before it was finished and when it was tested it was too salty to use. It took another year of adding more compost and leaching it with water to make it useable.
Happy gardening in Hawaii. Gardens are where people grow.