If a boat moves along with the water stream, then the net speed or the effective speed of the boat will increase and it said to be downstream.
True
Consider, a boat moving at a speed of S1 and the speed of the water stream is S2.
If the stream and boat are moving in the same direction(Downstream or along the stream), the Effective speed of the boat = S1 + S2, means that speed of boat will increase
And, If the stream and boat are moving in the opposite direction(Upstream or against the stream), the Effective speed of the boat = S1 - S2, means the speed of boat will decrease.
So, the given statement is true.