Simulation is essential for marine robotics, yet existing platforms often trade off visual realism, experimental throughput, and ocean conditions derived from real data. We present OneOcean, a data-grounded simulation suite and benchmark that combines bathymetry, currents, and tides in a shared spatiotemporal dataset. Optional pollution fields support pollution-response evaluation. OneOcean defines ten tasks covering navigation, sensing, coordination, and pollution response, with teams of 2–10 vehicles. The simulator supports three to six Degrees of Freedom (DoF) and records standardized metrics and run configurations across tasks, difficulty tiers, and team sizes. Experiments across dataset variants and underwater scenes compare heuristic control, Behavior Cloning (BC), and Large Language Model (LLM)-assisted planning under ocean currents.