A 20-second exposure will capture the reflection of the stars, but you may pick up movement on the water that reduces the clarity.
You may find you need to balance your shutter speed a little here, depending on the conditions. A 20-second exposure will capture the reflection of the stars, but you may pick up movement on the water that reduces the clarity. Try shorter exposures for your reflection shot – but it might mean you have to work harder to bring out the stars in the edit. Lightroom's linear-gradient edits are perfect for bringing out the clarity and sharpness of reflections, so give that a go.