Full paragraph for context: re: muddy v. clear(er) water
We need to cut the Standard Graduation Speech [to high schoolers] down to, "what someone
else with your abilities can do, you can do; and don't underestimate
your abilities." But as so often happens, the closer you get to the
truth, the messier your sentence gets. We've taken a nice, neat (but
wrong) slogan, and churned it up like a mud puddle. It doesn't make a
very good speech anymore. But worse still, it doesn't tell you what to
do anymore. Someone with your abilities? What are your abilities?
He argues that 'don't give up on your dreams' should be 'don't give up". He uses the example of a high school student of diminutive size and physical prowess being advised to aspire to professional sports (an unrealistic dream). To un-muddy the waters (water = the message to high school graduates) the advice should be: don't give up.
The author has taken a pithy slogan and added several qualifications,
which makes it a lot more accurate but less inspiring. StoneyB