Larger transmitter currents and lower receiver self-noise are limitations of deep marine controlled-source electromagnetic (CSEM) prospecting. Therefore, we developed a novel towed CSEM system using double vessels that has variable offsets between the transmitter and receiver. This system provides high efficiency, high horizontal resolution, and greater penetration depths compared with the fixed-offset towed CSEM system, which is a key advantage of the system. Two vessels were used for the transmitter and receiver. The source-receiver offset can be arbitrarily adjusted according to the target depths. Additionally, all six orthogonal EM component measurements can provide more abnormal information for deep targets. We also developed a near-seafloor-towed CSEM receiver that contains a deck user terminal, master node, slave nodes, tail buoy, and neutrally buoyant towed cable. The receiver uses Ethernet and optic fiber communication to conduct real-time data transfer to aid in deck data quality checks, a fiber transfer time service to the submarine instruments that avoids time-synchronous errors, and an ultra-short baseline (USBL) to improve navigation position precision. During an offshore experiment that conducted gas hydrate mapping in the South China Sea, the towed CSEM receiver continuously acquired all electromagnetic components and status information, achieving a preliminary prospecting result. This towed CSEM system can also be expanded to marine-induced polarization and self-potential methods for data acquisition applications.