What is the best way to accurately measure wavelength between crests on a pier?
One simple way is to measure the period $T$ by timing the arrival of each  crest and estimate the wave speed $c$ from the depth of the water column $d$. Then one can compute a wavelength $\lambda$. 

Assuming the waves are shallow-water waves ($d/\lambda &lt; 1/11$) and linear (modeled by a sine or cosine function), the error should be no more than 10% (is that accurate enough? It gets more complicated otherwise).

Since we assumed these are shallow water waves they are nondispersive and the speed is simply $c = \sqrt{gd}$, where $g$ is the gravitational acceleration.

Thus, the wavelength is
$$\lambda = c\ T=\sqrt{gd}\ T\,.$$