While underwater acoustic sensor networks (UWSN) are still considered to be in their development phase, professional users are starting to understand their future potential. For example, large areas encircling offshore oil installations need to be environmentally supervised. UWSNs consisting of a large number of sensor nodes can present a viable solution to this challenge. One way to build such networks is to divide the large area into multiple small clusters of sensor nodes, not very different from cellular mobile networks. Given a certain sensor node density, the question is then to determine the cluster sizes that provides optimum capacity or minimum number of high-cost sink nodes. This paper contains such a scalability analysis for underwater sensor networks, and presents a set of analytical expressions giving the relationship between underwater acoustic communication ranges and obtainable sink and sensor node densities in clustered underwater acoustic networks. Two propositions are presented that states a linear relation between cell capacities and MAC efficiencies, and how this capacity is reduced by inter-cell interference in a hexagonal cell topology. The propositions are validated using simulation experiments in WOSS/ns-miracle.