This paper presents an ocean dynamical core in Oceananigans designed to make mesoscale-resolving simulations practical on GPUs. It describes the hydrostatic formulation, numerical implementation, and hardware-aware strategies behind the model’s performance. A near-global simulation at 8-kilometer horizontal resolution advances about a decade per day on 64 NVIDIA A100 GPUs. That result illustrates how improved computational efficiency can reduce reliance on approximate representations of ocean eddies. Readers get concrete performance benchmarks and implementation lessons for building and evaluating high-resolution ocean simulations on modern GPU systems.