It makes weird sense thematically. Essentially, if Steven and Co. find another corrupted gem, then he can make a tonic that'll heal them up.
But yes, this is essentially Bath Water the 2nd: Electric Boogaloo.

What? Noooo, it's just their bath water! That's not weird! The Diamonds checked it out on the Internet, and humans were already selling their bath water! So they gave some to their sister/nephew, it's cool.