It was a huge mass of jellyfish! Those "bubbles" in the water seen in the pictures below- are not bubbles. They are jellyfish floating on the surface.
There were jellyfish being washed up and scattered all over the sand.
Which seemed to make the seagulls very happy.
Fortunately the jellyfish invasion mostly cleared up within an hour or two and the water was once again safe for the human ocean lovers.