No problem
What I wrote was "Will the excuse that I've been very busy hold water?". So my excuse is that I've been very busy. Now let's examine the sentence.
"Will the excuse [that I've been very busy] hold water?"
So my question was "Will the excuse hold water?" and the idiom is "to hold water" meaning is it valid, is it acceptable.
Will the excuse hold water? = Is the excuse acceptable?
Does that help?