[velma]

Hamsters don't drink very much compared with other animals because of their natural habitats and robos being see tiny appear to drink very little so it can be hard to tell.

A couple of times I've found that the ball valve got stuck in the bottle so they couldn't actually get anything out. Some bottles work better than others and with one particular bottle I had to loosen the cap just a little bit to allow the water to flow. What sort of bottle do you have? Smaller bottles can be better as smaller spouts are better for robos generally, I think. The classic bottles that you get in most pet shops are fine. I personally like the ferplast drinkie (I didn't like the sippy though) although when Hector was old I swapped back to a small classic bottle to make it easier for him.
So check the bottle as your first port of call. He may have been thirsty from a non-functioning bottle and drank extra from his bowl and now back to normal amounts. (Unless I've misunderstood and he's only ever had a bowl with you) Or he may not be drinking enough if he doesn't like the contaminated water. He may be getting enough from the cucumber on those days he has it.

Is the rest of his behaviour the same as normal? Hard to tell I know as you only got him a week ago but is he alert? Are his ears up or flat to his head compared to normal? Is he eating normally? Keep an eye out for those things as an indicator if something isn't right.