  It remains to be ascertained whether sub-Neptune exoplanets primarily possess
hydrogen-rich atmospheres or whether a population of H$_2$O-rich "water worlds"
lurks in their midst. Addressing this question requires improved modeling of
water-rich exoplanetary atmospheres, both to predict and interpret
spectroscopic observations and to serve as upper boundary conditions on
interior structure calculations. Here we present new models of
hydrogen-helium-water atmospheres with water abundances ranging from solar to
100% water vapor. We improve upon previous models of high water content
atmospheres by incorporating updated prescriptions for water self-broadening
and a non-ideal gas equation of state. Our model grid
(https://umd.box.com/v/water-worlds) includes temperature-pressure profiles in
radiative-convective equilibrium, along with their associated transmission and
thermal emission spectra. We find that our model updates primarily act at high
pressures, significantly impacting bottom-of-atmosphere temperatures, with
implications for the accuracy of interior structure calculations. Upper
atmosphere conditions and spectroscopic observables are less impacted by our
model updates, and we find that under most conditions, retrieval codes built
for hot Jupiters should also perform well on water-rich planets. We
additionally quantify the observational degeneracies among both thermal
emission and transmission spectra. We recover standard degeneracies with clouds
and mean molecular weight for transmission spectra, and we find thermal
emission spectra to be more readily distinguishable from one another in the
water-poor (i.e. near-solar) regime.
