Some ocean waves glow electric blue when disturbed — that's algae defending itself

During certain 'red tide' blooms, the ocean fills with dinoflagellates that flash blue-green light the instant a wave, a swimmer or a predator disturbs them. The light isn't decorative: it's thought to work as a burglar alarm, startling would-be predators or drawing in bigger fish to eat whatever is threatening the plankton. The same species that light up the surf by night are often the ones tinting the daytime water reddish-brown.

— Wikipedia contributors, Bioluminescence — Dinoflagellate bioluminescence