What is a "current drag"? sub-surface current a current moving opposite to ship motion a device to measure sub-surface currents a device to measure surface currents?

The correct answer should be the third one you listed. The "current drag" then is a device that is used in order to measure the sub-surface currents. This device and the measurements it provides are extremely important for the field of oceanography which studies the aspects of the oceans both the physical and the biological ones.

Current drag is a device that is used to measure the sub-surface currents. Sub-surface currents are the peculiar type of current that is associated with oceans which flows under the normal surface currents. They are mainly determined by density of the water and the temperature aspects.