Our homeworld orbits a large red giant star. Normally, it does not output much radiation, so local flora evolved a dark red hue to absorb as much energy as possible.
Occasionally, the star undergoes spikes in increased flare output (colloquially known as "emissions"), which output much higher radiation doses than what the plants' biology requires. To protect themselves, they have specialized ultraviolet-sensitive organs to predict when a next emission occurs, which they shield themselves with via self-withering and shell-like leaves. Aquatic plants aren't too special in this case, the water protects them most of the time.
Nevertheless, it made us kind of famous for our world's colors.