In a book that I'm reading there is a sentence like this
If we focus just on our local galaxy, we know that there are about 100 billion stars, with about 20 billion Earth-like planets. Twenty billion makes for a lot of petri dishes for creating life.
So the numbers seem encouraging, but now we get into more difficult waters: How many livable planets actually have life?
My question is what does get into more difficult waters mean?
Please explain to me.