The bath is too hot so I'm adding cold water to cool it down.
Literal
Bath [subject-が] too-hot so water [object-を] add lukewarm-make.
水を埋めてぬるくする — 埋める here means 'to mix in/add (cold water),' and ぬるくする means 'to make lukewarm.' This use of 埋める for mixing in cold water is a traditional bathing term.