Ocean waves have long been a hazard to marine operations, making water wave isolation critical in ocean engineering. Traditional methods suffer from large area requirements, high costs, and incomplete isolation. We design a water wave isolation device using periodic gear arrays (PGAs), which overcomes these drawbacks and achieves perfect water wave isolation across a wide frequency band. Water wave isolation occurs by effectively creating a negative water depth, which fills a major gap in the field of manipulating water waves using metamaterial concept. We demonstrate the water wave isolation of PGAs through analytical methods, simulations, and experiments, proving that effective negative water depth is key to achieving the isolation of water waves. The PGAs perform well across a wide frequency band, with potential applications in port and ocean wave isolation. This discovery enriches water wave manipulation techniques and advances the development of water wave metamaterials.