In Japan where I was doing "kelp work" years ago one summer, it's known as kombu. Grandpa and son (in the family that I was living/working with for a while) would get up at the crack of dawn and go out in a small open motor boat. One person would man a 10-15 long pole with a few t-shaped handle on one end and prongs on the other end. With a box-shaped mask made from plywood and a pane of glass, the person would bend over the gunwhale with the mask apparatus on his face, with the glass pane side on the water, allowing him to see down through the water. With his free hand, he would maneuver the pole down to the seabed, using the prongs to grasp the "roots" of the kelp on the rocks, and try to pry it free. It was very, very laborious work to do this, it seemed to me...and then you had to haul up the 20+ foot long piece of wet, slippery and very thick kelp. Once it was brought back, each long kelp plant was scrubbed of barnacles, etc. and then laid out to dry for hours and hours. Then it was cut by hand and rolled by hand to be shipped somewhere. Small-scale, labor-intensive aquaculture, done in the summer just prior to salmon fishing season.

This pic gives an idea of how the kelp is retrieved in small boats: http://www.nymtc.com/pl_mtcpremium/images/200709makomubu-hama2.jpg