The Zuiderzee area in the Netherlands is a former inlet sea at the heart of the crossroads of three major regional languages. While these regional languages are largely distinct, previous work by the dialectologist Kloeke indicated similarities due to contact over water, notably the realisation of the Proto-West Germanic vowels *& amacr; and *a. Using various dialectometric methods, we analysed the distribution of these vowels for 121 localities in this region. Specifically, we tried to determine the dialectal landscape more thoroughly, find instances that illustrate cultural diffusion and migration, and evaluate the overall relationship between distance over water and vowel variations. Using a Bayesian population genetic method, admixture, we distinguished nine linguistically explainable clusters, demonstrating its potential. Moreover, we found evidence of cultural diffusion conforming to the overall presence of three different regional languages. Additionally, we employed the so-called matrix method in linear-mixed effects regression to demonstrate that the geographic distance helped to explain the geographic patterns of vowel variation. The distance over water was as effective a measure as the distance over land. We expect this to be common in areas with a history of intensive and sustained shipping traffic.