Given data

• Speed of swimmer in still water: $v_s = 4\,\text{km h}^{-1}$
• Width of river: $d = 1\,\text{km}$
• Downstream drift: $x = 750\,\text{m} = 0.75\,\text{km}$

Direction of swimming The swimmer makes strokes normal to the flow of the river.

So, relative to water:

• Across the river component = $4\,\text{km h}^{-1}$
• Downstream component comes only from river speed, say $v_r$

Time taken to cross the river Since the swimmer's effective speed across the river is $4\,\text{km h}^{-1}$, $t = \frac{\text{width}}{\text{across speed}} = \frac{1}{4}\,\text{h}$

Downstream drift relation In this time, river current carries him downstream by $0.75\,\text{km}$.

Hence, $x = v_r t$ $0.75 = v_r \cdot \frac{1}{4}$

Calculate river speed $v_r = 0.75 \times 4 = 3\,\text{km h}^{-1}$

Final answer The speed of river water is $\boxed{3\,\text{km h}^{-1}}$