Offshore wind energy is a rapidly expanding source of renewable energy worldwide, but many aspects of offshore wind turbine behavior are still poorly understood and are not accurately captured by low-cost engineering models used in the design process. To help improve these models, computational fluid dynamics (CFD) can provide valuable insight into the complex fluid flows that affect offshore wind turbine power generation and structural loads. This research uses CFD simulations to examine three main topics important to future offshore wind development: how breaking waves affect structural loads for fixed-bottom wind turbines; how platform motions affect power generation, wake characteristics, and downwind turbine behavior in floating wind turbines; and how rotor tilt angles affect wake characteristics when interacting with earth's surface. These high-fidelity simulations can help inform future improvements to engineering models like wake models, power prediction models, and breaking wave models, which are integral to designing and financing both offshore turbines and offshore wind farm arrays. First, breaking wave limits and slam force models are evaluated using CFD simulations of shoaling and breaking waves impacting monopile foundations, for environmental conditions representative of U.S. East Coast offshore wind sites. Second, floating turbine wakes are characterized by the velocity deficit, turbulent kinetic energy, and wake centerline location using large eddy simulations (LES) coupled via an actuator line model to the multidynamics turbine modeling tool OpenFAST. These wake metrics are compared for different floating platform types, atmospheric stability types, and environmental conditions. Third, the power generation of spar and semisubmersible floating turbines is simulated using OpenFAST with LES inflow, with different platform motions isolated. These power results inform a new analytical model for power generation in floating turbines. Fourth, downwind turbines with different platforms are simulated in OpenFAST using an upwind floating turbine's LES wake as inflow, to study how floating-turbine wakes affect a downwind turbine's power, blade loads, and towertop displacements. Finally, LES with an actuator disk model of a tilted wind turbine are performed for different tilt angles and blade-to-surface gaps, to characterize tilted rotor wakes and how they interact with the sea or ground surface.