You don't have to live near an ocean to gather "seaweed" for your own fertilizer. Any aquatic plants may be used, whether from freshwater ponds or the seashore. Seaweed may seem like unwanted waste when lying at the water's edge, but never gather the plants from private or public land without permission. Many municipalities restrict seaweed gathering or require a permit. Gather only those weeds that have washed up on shore so aquatic ecosystems are not affected. Seaweed farthest from the water line usually will be driest and leached from some of its salts. Never gather from sites known to be polluted.