Ok,first.Let's review the two moves.
PP 10(max 16)
Muddy Water has the same base power as Surf.But it has less accuracy
and PP.But it does have 30% chance of lowering accuracy.
PP-15 (max 24)
While Surf has more accuracy and PP.But doesn't have a side-effect.
Conclusion:I would go with Surf because of it's reliability.(But I
wouldn't go with it in double battles.)
hope I helped