Seaweed

Just to add to the fun

The foreshore, as exactly as Aunty says, the intertidal zone, but also includes the area of land between the wter's edge and cultivated or occupied land 

and as such is generally owned by somebody (mainy the Crown of course) who often allow free access to the public but not permission to remove seaweed, sand, stones etc

Some beaches even have notices up to say nothing may be taken away from the area 

Make your own minds up as to any offence committed