It has nothing to do with resonance. It's much the same principle as a pulley where you focus little force over a long distance into a lot of force over a smaller distance.
It's an oscillating system with a dramatically greater amplitude than its neighbours sharing the same driver. I'm having a hard time believing it has nothing to do with resonance.
Edit: Resonance may pay a tiny role, but it's a long way from the star of the show.
It also works with normal waves, storm surges, and Tsunamis which are non-oscillating systems. It's due to the large amount of kinetic energy in moving water which allows a much wider range of waves to benefit than you get with a resonance effect. Picture a large rock in a fast moving stream, the water bunches up in front of it and in some cases you can see a bump the water over some obstruction. http://ak.picdn.net/shutterstock/videos/6538901/preview/stoc...
PS: Because the land slopes up the water nearing the shore can have a slightly lower angel over a long distance which adds up. Thus, the total height at the end might be 20+ feet, but at the edge of the bay it's only vary slightly above sea level. However, the reverse at low tide does not work nearly as well because the slope dampens the effect vs reinforcing it. Resonance effects require both the height and trough of each wave to reinforce each other.
Picture a single wave entering a V shaped harbor. http://en.wikipedia.org/wiki/Bay_of_Fundy Now realize you can think of the tide is a very low frequency wave.