Seetang — Stardew Valley Angel-Guide

Seaweed is an item that can be caught using a Fishing Pole in the ocean during all seasons, including a 41% chance to catch Seaweed while fishing in the deep-sea submarine ride at the Night Market. It is never caught in Crab Pots.

It can also be foraged from a narrow strip of shore by the Tide Pools, i.e. to the east of the wooden footbridge. but it spawns relatively infrequently (3% chance per night). The saltwater crab pot fish (Cockle, Crab, Lobster, Mussel, Oyster, and Shrimp) may produce 1-3 Seaweed in a Fish Pond when the population of the pond is at least 1.

Catching Seaweed with a Fishing Pole always produces standard-quality seaweed and gives 3 Fishing XP. Picking up Seaweed from the beach gives 7 Foraging XP, with quality controlled by foraging skill and bonuses (including possibly iridium quality).

Seaweed is displayed in the Fish section of the collection tab, and counts towards the four fishing Achievements. However, it is otherwise not considered to be a fish, and therefore cannot be sold to Willy at the Fish Shop, does not benefit from fish price bonuses, and cannot be used in fish recipes.

It's also possible to find Seaweed in the "Dangerous Mines" on floors 1-39 when the Shrine of Challenge is turned on. This seaweed will always be normal quality.