Good barriers make good languages. Scholars have long speculated that geographical barriers impede linguistic contact between speech communities, and promote language diversification in a manner similar to the process of allopatric speciation. This hypothesis, however, has seldom been tested systematically and quantitatively. Here we adopt methods from evolutionary biology and attempt to quantify the influence of oceanic barriers on the degree of lexical diversity in the Japanese Islands. Measuring the degree of beta diversity from basic vocabularies, we find that geographical proximity and, more importantly, isolation by surrounding ocean, independently explains a significant proportion of lexical variation across Japonic languages. Further analyses indicate that our results are neither a byproduct of using a distance matrix derived from a Bayesian language phylogeny nor an epiphenomenon of accelerated evolutionary rates in languages spoken by small communities. Moreover, we find that the effect of oceanic barriers is reproducible with the Ainu languages, indicating that our analytic approach as well as the results can be generalized beyond Japonic language family. The findings we report here are the first quantitative evidence that physical barriers formed by ocean can influence language diversification, and points to an intriguing common mechanism between linguistic and biological evolution.