The term “seaweed” is used to describe a large number of plants and algae that live in and by the ocean. Categories of seaweed include different types (algae, kelp, sea grasses, etc) as well as colors (red, brown, green).
Seaweed , or macroalgae , refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae.
From Wikipedia: Seaweed
Although summer beachgoers may not find it fun to swim with or step in seaweed, it’s a very important part of nature. Seaweed provides homes and nutrients to sea and seaside critters. It produces the oxygen we need to breathe and also helps absorb carbon from the atmosphere.
Seaweed is also an important resource for humans - it is used as food, in medicine, in cosmetics, and as a commercial good. It can be grown quickly and sufficiently, without need for harmful fertilizer or pesticides.
Unlike land-based crops, seaweed doesn’t require fertilizer, pesticides, freshwater, or, obviously, land . It grows fast—some marine algae can be ready to harvest in as little as six weeks—and absorbs CO2 while it’s growing, making it a valuable carbon sink. Algae also absorbs other excess nutrients like nitrogen and phosphorus, and creates new habitats for marine life.
From World Wildlife Fund: Seaweed is a win for you, the ocean, and the planet
Even if you’re far away from the sea, if you look around you’re likely to still find seaweed! Seaweed is a popular ingredient in cosmetics and beauty products. It is also used as a natural thickening agent - did you know that seaweed is present in many kinds of chocolate milk? Look for “carageen” listed as an ingredient - if you see it, it means you’re drinking some seaweed goodness! 
Some of seaweed’s magickal associations are listed below. To learn more about associations and how they work in magick, visit Magickal Correspondences.
Seaweed Magickal Associations:
, Venus 
, Neptune 
Considering seaweed’s many mundane benefits for health, beauty, animals, the ecosystem, etc - it’s natural that seaweed has many magickal benefits as well!
Protection 
Seaweed provides a safe habitat for sea life to thrive in. The tangles of underwater seagrass or fields of kelp washed ashore provide dark space for creatures to escape from predators in. On a less visible level, seaweed also provides oxygen and absorbs carbon dioxide, helping creatures on land and sea to live.
Consider using seaweed in protective spellwork - dried and cleaned seaweed can be added to spell jars, spell bags, or included in ritual work.
Beauty & Self-Care 
Seeing soggy piles of seaweed on the beach may leave you thinking “yuck”, but look deeper - seaweed is a popular addition to cosmetic products for a reason! Abundant in nurturing minerals, seaweed is used to rejuvenate and replenish the body. In the ocean, seaweed glows with the sunlight, shines with it’s own range of beautiful colors
When you’re out of your element and feeling icky, considering adding some seaweed to your glamor, beauty, and self-care spells to wash away doubt and reveal your natural beauty 
Growth & Endurance 
Able to grow rapidly and in tough environments, seaweed is sturdy and sure. Consider adding seaweed to your growth, strengthening, and endurance spells. As a thickening agent, seaweed can help you tough out difficult situations and stay strong in the face of opposition.
Disclaimer: If you want to harvest seaweed for use, it is a good idea to check any local harvesting laws. Never take seaweed from contaminated or endangered waters. Not all seaweed is edible. Prior to use or storing, be sure to properly clean and dry it first - this helps to save any sea life living in the seaweed and will make the seaweed more shelf-stable.
