I provided water bottle to my opponent, he drank it then lost on time due to the need of using bathroom. Is this unethical? Many years ago, I was playing in a chess tournament. We had been at the board for at least three hours. My opponent had spent the entire time at the board trying to find good moves. I had the better position and plenty of time on my clock. My opponent had less then 10 minutes on his clock. I also taken a couple of bathroom breaks.
The tournament provided free bottles of water. I got up from the board and got two bottles of water. I started to drink one and placed the other bottle near him. Neither one of us said anything to each other. He opened his bottle and started to drink it. About 10 minutes after drinking the water, my opponent needed to use the bathroom which he did. While he was gone from the board, he went over on the clock.
Did I do anything unethical?
No, what you did is what is known as a gambit. You offered your opponent material (in this case a water bottle), which he accepted. This would be considered a blunder on his part, since although it gave him a temporary hydration advantage, it ended up costing him time due to the uncomfortable situation it caused for his bladder, which needed to be resolved. If he had instead declined the gambit, he may have won, but such is the way of the game.