Seawater intrusion modeling is challenging either because of a lack of many approaches' ability to cope with complex environments or significant computational expense. Seawater intrusion models also face issues with parameterization and validation due to problems with many parameters and also a lack of observational data. Bakker and Schaars (2013) proposed a method to solve for the steady-state interface using a single-density flow model (MODFLOW). In this thesis, this method was applied to 3 existing coastal flow models and shown to be able to compute the steady interface. The work also contains examples of applying critical pumping solutions to the models to demonstrate its applicability as a management tool. The research also contains an effort to solve for an interface when the saltwater is moving but was ineffective. Modifications may be made to resolve the issues with the latter approach that could lead to a advances in interface modeling methods.