If you live close to a public access beach, you may have access to one of the best fertilizers for your garden. Seaweed is a wonderful fertilizer and soil conditioner containing many minerals and trace elements that may help round out the nutrient profile of your soil and plants grown in it.
However, there are some concerns about the pollution level in the ocean water which may accumulate in the seaweed (although this is much less of a concern than the pollution level in seafood because toxic substances like heavy metals are stored in fatty tissues of animals and fish, where seaweed and kelp (a type of seaweed) are akin to fresh plant matter; the only difference is that it grows in the water instead of on land like lettuce and kale). There are also concerns about the disturbance of coastal habitats (especially if the seaweed has been sitting there a while, making it more likely that many critters have moved in and spawned in the seaweed pile) -- so be sure to harvest just the modest amount you need and not to over-harvest. Be aware, too, that in some municipalities, the harvesting of anything off the beach is regulated and may require a permit. Being mindful of these concerns, one may harvest a modest amount and make it go a long way by making concentrated liquid seaweed or kelp fertilizer as follows. You can of course directly put seaweed on top of your soil and let it break down over winter. In locations where summer is hot and dry, the seaweed should first be composted then worked into the soil, otherwise it would just dry up without breaking down.
First, stuff a barrel as full as you can with fresh seaweed. Then pour water in enough water to submerge the seaweed. Put a top on it and weigh it down to keep the top on. It's best not to use an airtight lid, as the aerobic process (decomposition with oxygen) is generally more pleasant and less stinky than the anaerobic one (decomposition without oxygen). Then let the barrel sit; stir the contents from time to time (once a week or two would suffice). It make take just 2-3 weeks in the warmer months to break down into liquid fertilizer, or it may take 2-3 months if the weather is cold. It's done when the seaweed has emulsified (broken down completely and mixed well with the water, yielding a medium thick dark liquid). Dilute the resulting liquid seaweed fertilizer or liquid kelp fertilizer 20 to 1 with water before applying to plants as a booster once every two weeks during the growing season. You can also apply it as a foliar spray -- spraying diluted liquid seaweed or kelp fertilizer not only nourishes the plants but also discourages pests.
Making your own liquid seaweed fertilizer saves you money (they often go for upwards of $20 per 1 litre bottle in garden centers) and assures you of the purity of the product (most commercial liquid seaweed fertilizers are treated with ascorbic acid to stabilize the product and make it shelf stable). It's best to store the finished liquid seaweed fertilizer in a glass container with the lid a bit ajar to allow gases to escape in the maturing process.