Sea Lettuce (Ulva lactuca)

This is a membranous green algae with flat broad thalli and wavy to frilly edges. It looks like loosely arranged leaves of a lettuce. The thalli (algal body) are only two cells thick making them look delicate and transluscent.
These were seen in rock pools in tidal zones.
When fresh, this algae can be used in soups as is done in some European countries.