This paper examines how interdisciplinary hydro-economic analysis can inform seawater intrusion management and analyzes how differences in aquifer type (confined or unconfined) lead to differences in spatial-dynamic marginal seawater intrusion cost externalities and associated differences in optimal management policies, which have the greatest potential for large economic gains. By numerical maximization of the present value of net benefits of a groundwater resource linked with a finite-difference seawater intrusion model, we calculate the spatial dynamic profile of optimal groundwater extraction in an unconfined aquifer with seawater intrusion. Comparison of the resulting spatial-dynamic profile of marginal pumping cost and seawater intrusion externalities between aquifer types, using similar geometry and revenue functions as previous numerical and analytical economic research on confined aquifers, demonstrates that the confusion in the economic seawater intrusion literature about the optimal spatial structure of pumping fees can be directly attributed to differences in aquifer type.