OpenFAST, an open-source wind turbine simulation tool developed by the National Renewable Energy Laboratory, is being expanded to model fixed and floating marine turbines. Since OpenFAST was originally developed for wind turbines, its models are limited to simulating rotor operation in air and are restricted to certain geometries. Development for marine turbines is focused on capturing additional physics relevant to operation in water and enabling the simulation of more generalized support structure geometries. New functionalities are grouped into rotor, substructure, and inflow categories. Expansion of the rotor and substructure hydrodynamic models involves simulating additional loads caused by buoyancy, added mass, and inflow accelerations on the blades, tower, hub, and nacelle. Added mass loads due to hydroelastic structural deformation, structure motion, and blade pitch acceleration are included. Models that simulate the interaction between the rotor and tower are also being modified to capture tower shadow and dam effects from multi-member support structures and more generalized member shapes. Furthermore, hydrodynamic lifting loads on faired members are calculated using a pre-defined table of lift coefficients, similar to the approach used for existing drag calculations. A check to detect the presence of cavitation on the blades was previously added to the rotor hydrodynamic model. Functionalities added to the inflow models include superimposing wave and current velocities in the flow upstream of the rotor and calculating inflow accelerations caused by turbulence. The rotor, substructure, and inflow models are also being adjusted to allow the tower and rotor to be defined below the mean sea level. Additional functionalities that are not being actively developed, but are areas of potential future work, include modeling multiple rotors on a single platform, accounting for flow confinement effects from multiple rotors and from the surface/seabed, expanding the aeroacoustics model for underwater operation, modeling biofouling on the rotor, and simulating other marine topologies, such as cross-flow turbines, kites, and oscillating hydrofoils. To aid in code development and provide a set of example cases and baseline results, reference models are being developed for fixed and floating axial-flow marine turbines. Although field-scale reference models are being developed to align with an existing experimental measurement campaign, future work includes the definition of reference models across a wider range of scales. The OpenFAST development detailed here is supported through two different projects. One of these projects is the Powering the Blue Economy (PBE) initiative funded by the U.S. Department of Energy (DOE), which seeks to develop technologies for applications other than utility-scale electricity generation. These applications often have smaller power requirements and employ smaller devices. The capabilities being added to OpenFAST for marine turbine modeling are relevant over a range of scales, and code validation at smaller scales will ensure the tool can be used for PBE applications. OpenFAST is also being developed for marine turbines as part of a larger project administered by the DOE Advanced Research Projects Agency-Energy. This project is building an open-source, control co-design tool for marine turbines and includes models across a range of fidelities, with OpenFAST being the highest-fidelity model included in this tool.