I find ChatGPT is good for this sort of thing (+ fact-checking, as always).
Here's what I got - I also ran it through a few times and it recommended a similar phrase each time, which is usually a good sign.
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Using "水分補給" (Suibun hokyū) on its own can work as a concise slogan. It directly translates to "Hydration" or "Fluid replenishment," conveying the message effectively in a succinct manner.
For a full phrase, you can try "水分補給を忘れずに!" (Suibun hokyū o wasurezu ni!) which translates to "Don't forget to hydrate!"