Sea lettuce looks exactly like its name: ruffled, brilliant green leaves that grow in tufts from shallow rocks. It’s usually found in calm waters. There are many species of sea lettuce in the Ulva genus, and as of right now I have no idea how to tell them apart–but from what I’ve read, that’s because they are all quite similar in looks and habit.
Sea lettuce is a green alga, and is edible though sometimes a little tough and bland. Some enjoy it for salads. One warning if you are planning to eat some Ulva is that it grows particularly profusely in water that’s been “enriched” by sewage or other pollution. So if you see a dense patch of the stuff, take a close & skeptical look at the nearby landscape before you harvest it…
Flowers Of Marin 