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:
- 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)
- 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)
- グイグイ飲もう！ "Let's drink lots!" (Might imply alcohol, but that might be funnier? or ぐいぐい)
- 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!")
- 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!
Oh... I'll just use 括弧（かっこ） for now, then. It's easier to type out anyway. Still a cool feature!