definitely water weight bro. you should be able to feel, maybe even see your muscles swell excessively during intense workouts, some more-so than others of course.
very important to stay hydrated when supplementing with creatine. dehydration causes all kinds of problems, all non-conducive to your training.
in about 4 -6 weeks you can expect to have gained around 10 - 15 lbs, assuming you have a solid workout and your diet is sufficient in calories and protein that is.
i have a few tips for mass gain on the 'Before You Get Started' page of the website below if you're interested