Holy Cow. A whole bottle in two days! That's nuts. Isn't there a way you could just put 10 drops or so into the well? Orange is probably no big deal but there are some EOs that you just wouldn't want that much of in the air. I have a more traditional style diffuser and use about 10 or so drops and then run it for a half hour here and there. OK, just looked at this thing closer and it appears there is no well, you have to attach your bottle to it. Gotta say I don't care for that design at all, it takes away some of the control. I love to diffuse a blend of grapefruit and peppermint and with a well diffuser you can just put a few drops of each in the well. With this diffuser you would have to put them in a different bottle to do that. Boo, frowny face. How about putting just a little of the EO in another bottle so the machine doesn't have the opportunity to suck up an entire bottle at once. Or, just run it for 15 minutes at a time a few times a day.
Sorry, for the long rant. I would be very upset about wasting even a cheap bottle of EO in such a short period of time. I'm pretty stingy with mine.
As for the water, I would check the instructions. I seem to remember mine recommending not adding water. Yours is way different though so who knows but it would really suck to mess up an expensive piece of equipment like that if it's not meant to have water in it.