Yep - I have my rear brake on the left as well ( as do most UK bikes) but even if I didn't I'd still reach with my right hand. Even though I'm right handed I'm much better at one-handed-riding with my left steering. If I tried to reach for my bottle with my left hand I'd get in a total mess and probably drop it!

Generally reach for my bottle whenever I'm thirsty, although occasionally I ride along with it in my teeth for a minute if the road suddenly requires 2 hands, till I'm out of danger.