A diagram of ocean layers can make light appear to end at a sharp horizontal line. Real underwater conditions need a more careful reading. NOAA explains that most sunlight is absorbed near the surface and that significant light rarely reaches beyond about 200 metres, although some light may be detected deeper. See its ocean-light overview.
Read the labels before the colours
When studying a diagram, check whether it presents broad zones or measurements at a particular place. A coloured band may be a teaching device, not a photograph of a fixed boundary. Note the units and any qualifications about water conditions.
Do not use the apparent brightness of a shoreline photograph to estimate how far light penetrates. Camera settings, viewing angle, and conditions near the surface complicate that inference.
Turn the diagram into a question
Ask what information would be needed for a local comparison: location, depth, time, and a stated measurement method. These details connect a general explanation to an actual observation.
For a notebook, write one supported fact from the source and one question the diagram cannot answer. This keeps a useful introductory model from becoming a claim that every part of the ocean has the same light conditions at the same depth.