A model is proposed for estimating water depths from thicknesses of pro-grading coarsening-upward barrier island and deltaic sequences. The prograding barrier sequence consists of a basal lower shoreface mudstone overlain by middle shoreface interbedded mudstone and sandstone, capped by a sandstone complex of beach-upper shoreface and coastal dune. Sea level coincides with the base of accretionary cross-stratified foreshore beach sandstone. Sediment thickness from this sea-level datum to the base of the lower shoreface mudstone is the same as water depth into which the barriers prograded. The prograding deltaic sequence consists of a basal pro-delta claystone overlain by delta-front interbedded sandstone and mudstone, capped by delta-plain sandstones and other facies in which sea level can be recognized. Sediment thickness from the sea-level datum to the base of the pro-delta claystones is the same as the water depth into which the deltas prograded. Constraints on the model are compaction, subsidence, and transgressions, but these are not a difficulty, provided the method is applied with care. Preliminary data indicates approximate water depths of 2.5 to 12.1 m for Pennsylvanian and Cretaceous barriers, 10 to 24 m for Pennsylvanian and Cretaceous cratonic deltas, and 31 to 89 m for Carboniferous basinal deltas.