Ulva lactuca, Sea lettuce (sometimes known as sea spinach or sea cabbage), grows in oceans around the world and is probably the most widely distributed of all edible seaweeds. The variety found in the N.E. Atlantic is considered by some to be a different genus, Ulva fenestrata but agreement on that is absent.
Whatever we get in Scotland is used in salads and in soups, whereas in Korea (both) it is used to make a version of 김치 – kimchi.
I’m told it is also popular in Hawaii where it may also appear in soups or salted with raw fish. Etc.
In China, where it is known as 海莴苣 (hǎi wō jù, sea lettuce) or in the packets illustrated here 海菠菜 (hǎi bō cài, sea spinach) or 石莼 (shí chún) it leans more to the Korean treatment. This lot comes from near the North Korean – Chinese border. It is pickled with spices and sold as a snack food.
Less often, I see it fresh.