If the machine is the BEI kind (using electric with handles), yes. Even if you drank a 1.5 litre of plain water can increase your fat percentage. BEI is greatly depended on amount of water inside our body. So, the best practices would be:
1. Drink a cold 1.5 litre of plain water (you need to drink the same amount of water with the same coldness everytime you want to take measurement)
2. Take a piss and a dump.
3. 30 minutes after step 1, then start measuring.