Below is a picture of a swimmer in a river. looking at the vectors being added is the swimmer going downstream or upstream? downstream, the current is the swimmer go faster relative to the land. upstream, the current is slowing the swimmer down.

Downstream, the current is helping the swimmer go faster relative to the land.

Explanation:

As shown in the figure the resultant vector is the addition of velocity of water and velocity of swimmer.

This shows that the swimmer is in the direction along the flow of the current. Thus, answer is downwards, the current is helping the swimmer go faster relative to the land.