The sea lettuce (Ulva lactuca) can be found all around the British Isles and can be seen on all kinds of shore lines from sandy beaches to rock cliffs, from shallows to deep water. Most frequently it will be seen as fragments washed up on the tide strand line. It grows in large in large drifting forests.
The sea lettuce has broad but very thin 'leaves' which are bright green, the stipe which attached it to its footing is very short. If thoroughly cleaned sea lettuce can be eaten raw just like lettuce you might grow in your garden or buy in the supermarket but, of course, it is not related. It was just named sea lettuce because it is green and edible. It can be cooked and used to make soup, or can be served boiled with fish or meat. In some areas of the world it is harvested and used as a crop fertiliser. A versatile plant to have around even if you rarely see it!