Sometimes, but mainly in the first few days and mostly inside the muscles. A 2021 evidence review by creatine researchers in the Journal of the International Society of Sports Nutrition says water retention is the most commonly reported side effect, seen early on, especially with a high “loading” dose of about 20 g a day; because creatine is pulled into muscle cells along with sodium, water follows it there. Several training studies lasting roughly four to ten weeks, including ones that followed a loading week with about 5 g a day, found no significant change in total, intracellular or extracellular body water, and in a study where body water did rise it was mainly intracellular and kept pace with gains in muscle mass. The review concludes that any increase is mostly short-term and that creatine “may not lead to water retention” over longer periods; in other words, extra water tends to be stored in the muscles rather than showing up as general bloating.
Image: “Creatine monohydrate supplement” by Apinanaivot, CC0, via Wikimedia Commons.