A single sea-temperature number can be useful, but it has an address: a place, a time and a depth. The water beneath the surface need not share that value. A vertical profile makes the missing dimension visible.
Read down the water column
In a temperature-versus-depth diagram, check which axis carries depth and whether it increases downward. Follow the line from the surface instead of assuming its shape works like a time-series graph. The line describes measurements or a model across depths, not the route of one travelling parcel of water.
Find the transition rather than a universal depth
NOAA explains a thermocline as a transition between warmer surface-layer water and cooler deep water, with temperature changing rapidly through the transition. A teaching diagram illustrates the idea; it does not establish the exact structure at every coast on every day.
Try an invented profile with several shallow readings close together in temperature and a sharper decrease farther down. Circle the section of rapid change. Label the numbers as illustrative so the sketch cannot be mistaken for a local observation.
Keep the sampling context beside the picture
When reading real data, record location, collection date, units and depth range. If only the surface was measured, describe the surface. Do not fill the deeper water with assumptions because the graph has empty space available.
This complements thinking about changing light below the surface. Both invite you to stop treating the ocean as one uniform container. The useful next question is often not “how warm is the sea?” but “where in the sea was that temperature measured?”