There is an entire crafting system in Disney Dreamlight Valley. That means you will need to gather materials around the valley in order to craft your own furniture and craft items for the various residents to complete quests. While some of the crafting materials are very abundant, some are either scarce or quite hard to find. Seaweed is one of the more uncommon materials that you will find, but it doesn't seem that way at first. Here is how you can get Seaweed in Disney Dreamlight Valley.
You may not notice any Seaweed in your world until you unlock Dazzle Beach. This will happen as part of the story once you have restored the Pillar of Friendship. When you go down to the beach, you will most likely spot a few clumps of Seaweed along the edge of the water. You can find Seaweed before this as well if it happens to be on the edge of the ponds in the previous areas.
Once all the Seaweed on the beach is gone, it will not respawn. This is unfortunate if you need more of it to complete a project. Luckily, it is actually pretty easy to obtain but it might take a while to get a lot of it. To get Seaweed, simply use your fishing rod and cast your line out into the ocean or in any part of a body of water where there aren't ripples.
Related: How crafting works in Disney Dreamlight Valley
Leave your line in the water until it says you have a bite. If it is Seaweed, it will come up very easily. Keep repeating this process until you have enough for your current project. Seaweed is needed to make fiber which is a material required for many different crafting projects.