Why do oceanographers take measurements of the conductivity of the seawater?

Because conductivity is useful to measure salinity in the water

Explanation:

One of the most important things in the oceanic ecosystems is the water salinity because it's necessary an equilibrium in this salinity (amount of ions) for the stability of the ecosystem. For that reason, oceanographers and other scientists need to measure this water characteristic.

Conductivity is the ability of a material to conduct electric current through disolved ions. These ions in water change its salinity, if there are a lot of disolved ions in water it is vert salty and vice versa and if water ir very salty so it is too conductive.