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!”

• ChatGPT 3.5

I’d suggest asking on hinative.com, since you can ask a native speaker there.

水分補強 sounds like “water rations” or something to me (dictionary). It’s just the nouns and doesn’t imply “stay hydrated!” to me, personally - like writing “Hydration” on a bottle.

Offhand, I’d suggest asking a native speaker about some of these:

1. 熱中症にご注意！ “Be careful of heatstroke!” (commonly said phrase to imply “drink water” and cool off, probably fits with skeleton best I think? Unless it’s winter, definitely more common in summer)
2. 水分を忘れないでね！ “Don’t forget to hydrate!” (I need to ask a native speaker if the “ne” makes it sound feminine though)
3. グイグイ飲もう！ “Let’s drink lots!” (Might imply alcohol, but that might be funnier? or ぐいぐい)
4. のんで、のんで、のんでのんで、のんで 、のんで、のんでのんで、のんで ! “Drink!” x10 (Might also have alcoholic connotations - like “Chug chug chug!”)
5. 水分補給しよう！ “Let’s hydrate!” (Or しましょう which is formal/polite)

I think the first two work best with a skeleton. Maybe 2 is clearest.

Not sure what other ideas people might have - also note that I’m NOT a native Japanese speaker. Just throwing out some ideas!