Due to the difficulty and expense of underwater field trials, a high fidelity underwater simulator is a necessity for testing and developing algorithms. To fill this need, we present HoloOcean, an open source underwater simulator, built upon Unreal Engine 4 (UE4). HoloOcean comes equipped with multi-agent support, various sensor implementations of common underwater sensors, and simulated communications support. We also implement a novel sonar sensor model that leverages an octree representation of the environment for efficient and realistic sonar imagery generation. Due to being built upon UE4, new environments are straightforward to add, enabling easy extensions to be built. Finally, HoloOcean is controlled via a simple python interface, allowing simple installation via pip, and requiring few lines of code to execute simulations.