This guide explains how consecutive fishing in Pokemon X and Y raises the probability of catching a Shiny Pokemon while fishing for long and uninterrupted periods of time with any rod.
You will always catch a Poliwag if you use an old rod at Route 16. This is because it is a 100% encounter rate when you fish there with an old rod. Poliwag will be at Level 15 so it should be quite easy to actually catch it with a Dive Ball or Net Ball
Running away or causing the Pokemon to faint does not increase your chances of catching a Shiny Pokemon but neither does it decrease them.