Why is the ocean blue?

The ocean looks blue because water absorbs more of the red part of sunlight and leaves more of the blue part for our eyes to see.

That is the basic reason the open ocean looks blue, but the exact shade can shift. Floating sediment, algae, and other particles can push the water toward green, brown, or even red in shallow or coastal areas.

Depth matters too. In clear open water, red light gets absorbed first, so the remaining light is bluer the deeper you look. Near shore, suspended particles and bottom reflection can overwhelm that effect, which is why a beach cove can look green or muddy while the open sea looks dark blue.

That is also why two stretches of ocean on the same day can look completely different. A clear offshore patch can look cobalt, a bay with algae can look green, and a stormy or sediment-heavy coast can look gray-brown even though the water is still the same basic stuff.