Observations and theory often go hand in hand in natural sciences. There is a risk of false conclusions if one or another is missing. Observations are needed for the basis of theory and for strengthening them. In turn new theories create prerequisites for more accurate observations.
Like physicists work in their own laboratories, so do ocean physicists work in their own research facilities around the oceans of the world. The most important tool for an oceanographer, alongside with one’s brain, is a proper and seaworthy research vessel. A research vessel enables different measurements to be made with different devices that are lowered into the sea. Buoys anchored in the sea or floating free and other measurement devices are also used to do measurements. There can also be different devices at the shore, like mareographs that measure sea level, for example.
These instruments are used to measure waves, water level, currents, water temperature, salinity, dissolved gasses, light and other phenomena and features of the seas.
These days observing seas with different satellites is more common, but without observations from the sea surface their data is not very useful.