Recent discoveries of water-rich, sub-Neptunian to Neptunian-massed exoplanets with short-period orbits represent a new parameter space for the study of exoplanetary dynamos and planetary science in general. Here, we explore the theoretical geometry of the dynamo source region within this parameter space using 1-D interior structure modeling. We model planets with 4 chemically distinct layers that consist of (1) an iron core, (2) a silicate layer, (3) an H2O layer, and (4) an H/He envelope. By varying the total planetary mass the mass % of the H/He envelope and the equilibrium temperature, we can survey the parameter space for potential dynamo source region geometries. What we have found is that due to the nature of the phase diagram of water at pressure and temperature conditions of planetary interiors, small variations in equilibrium temperature and H/He envelope % can produce large interior structure differences in a water-rich exoplanet. The resulting difference can manifest itself in the planet’s external magnetic field morphology. This allows us to put constraints these parameters based on magnetic field signatures.