User blog comment:MrSolomon/MGQ Fanfic: War of the Seraphim/@comment-10534118-20150723070612/@comment-10534118-20150724050407

Slime monsters often are depicted as being able to grow through water. But cannot swim in large bodies of water like lakes, rivers or oceans due to them dissolving into the body of water.

They live off water usually through soaking in small pools like tubs and such. So it makes sense for a slime to drink water.