Creatine

good quality creatine should not cause any major water retention, few lbs maybe.

Buy creapure.

Very little. If you're noticing strange weight gain its probably due to something else...

Creatine is supposed to cause water retention. Seeing as how your muscles are mostly water, this is a good thing in this context. Take monohydrate, 5g/day.

This is from http://www.jissn.com/content/7/1/7. There's dosage at the end too.

Creatine monohydrate
In our view, the most effective nutritional supplement available to athletes to increase high intensity exercise capacity and muscle mass during training is creatine monohydrate. Numerous studies have indicated that creatine supplementation increases body mass and/or muscle mass during training [70] Gains are typically 2 - 5 pounds greater than controls during 4 - 12 weeks of training [71]. The gains in muscle mass appear to be a result of an improved ability to perform high intensity exercise enabling an athlete to train harder and thereby promote greater training adaptations and muscle hypertrophy [72-75]. The only clinically significant side effect occasionally reported from creatine monohydrate supplementation has been the potential for weight gain [71,76-78] Although concerns have been raised about the safety and possible side effects of creatine supplementation [79,80], recent long-term safety studies have reported no apparent side effects [78,81,82] and/or that creatine monohydrate may lessen the incidence of injury during training [83-85]. Additionally a recent review was published which addresses some of the concerns and myths surrounding creatine monohydrate supplementation [86]. Consequently, supplementing the diet with creatine monohydrate and/or creatine containing formulations seems to be a safe and effective method to increase muscle mass. The ISSN position stand on creatine monohydrate [87] summarizes their findings as this:

1. Creatine monohydrate is the most effective ergogenic nutritional supplement currently available to athletes in terms of increasing high-intensity exercise capacity and lean body mass during training.

2. Creatine monohydrate supplementation is not only safe, but possibly beneficial in regard to preventing injury and/or management of select medical conditions when taken within recommended guidelines.

3. There is no compelling scientific evidence that the short- or long-term use of creatine monohydrate has any detrimental effects on otherwise healthy individuals.

4. If proper precautions and supervision are provided, supplementation in young athletes is acceptable and may provide a nutritional alternative to potentially dangerous anabolic drugs.

5. At present, creatine monohydrate is the most extensively studied and clinically effective form of creatine for use in nutritional supplements in terms of muscle uptake and ability to increase high-intensity exercise capacity.

6. The addition of carbohydrate or carbohydrate and protein to a creatine supplement appears to increase muscular retention of creatine, although the effect on performance measures may not be greater than using creatine monohydrate alone.

7. The quickest method of increasing muscle creatine stores appears to be to consume ~0.3 grams/kg/day of creatine monohydrate for at least 3 days followed by 3-5 g/d thereafter to maintain elevated stores. Ingesting smaller amounts of creatine monohydrate (e.g., 2-3 g/d) will increase muscle creatine stores over a 3-4 week period, however, the performance effects of this method of supplementation are less supported.

8. Creatine monohydrate has been reported to have a number of potentially beneficial uses in several clinical populations, and further research is warranted in these areas.

I've been taught that caffeine adversely affects the ergogenic performance of creatine supplementation, and I believe it was from reference text books, but I have yet to find it published in a peer reviewed journal online (only a brief search so far).