## 1 Introduction  
With the rapid expansion of human living spaces and the demand for natural resources, ocean exploration is receiving growing attention [1, 2]. Various types of equipment have been created and applied, significantly expediting the resource development process and also driving progress in related research. CPNT technologies are among the most critical and indispensable technologies needed for maritime activities. The development of Global Navigation Satellite System (GNSS), including Beidou (BD), Global Positioning System (GPS), GLONASS, and Galileo, provides robust support for CPNT services for activities conducted above the ocean surface. However, the communication medium applied for terrestrial PNT (Positioning Navigation and Timing) application is electromagnetic waves, which rapidly attenuate underwater because of the conductivity and dielectric loss of water. However, water can efficiently transmit mechanical vibration energy, making acoustic waves, a kind of mechanical energy, currently the only option for medium and long distance underwater communication method. Due to the scarcity of available communication and positioning resources [3; 4], as well as the absence of a unified standard for CPNT systems, function and equipment are often designed separately, leading to a waste of system resources and low system effectiveness. With the objective of building a unified underwater CPNT system, we investigate the essential conditions for the underwater CPNT service. Real-time precise positioning is the basis of underwater operation and navigation application, and the timing service is a crucial guarantee for underwater collaborative operations. Specifically, an underwater communication network with a long range and high data transmission rate can efficiently transmit information. However, the underwater environment is intricate and presents several unavoidable challenges. * Since the magnetic wave, the main information carrier of the current PNT system, is unable to propagate far in water because of energy absorption, scattering, and seawater conductivity [5; 6], the traditional PNT system cannot work properly underwater. In addition, as a new application field, more systematic research and implementation of the underwater PNT system are needed. * Sound, a mechanical wave, has been demonstrated to be the sole medium capable of propagating over long distances underwater [7]. Several issues are associated with the acoustic channel, including restricted bandwidth, prolonged latency, dynamic time-space properties, and significant Doppler effect [4]. * Due to the low communication rate, the design of CPNT signals presents a considerable challenge to maximize the use of communication resources. * Assessment and performance boundary analysis for integrated multifunction acoustic network systems lack systematic theoretical support. * Given the high costs of sea trials, most analyses are simulation-based, which poses difficulties in accessing experimental data from actual sea trials. Based on the practical need for an underwater CPNT system and the aforementioned challenges, it is worthwhile to delve deeper into investigating the feasibility of a practical and efficient underwater CPNT system. In this paper, we are dedicated to developing a \(U^{2}CPNT\) framework to fully leverage the limited underwater communication and energy resources to support the development of the underwater CPNT system. Several original research studies have been conducted in this paper, which are outlined below. * The design of the unified system architecture for the underwater CPNT system has been carried out. * A CPNT network performance analysis model based on the Cramer-Rao bound was established. * The improved performance of the \(U^{2}CPNT\) network system in CPNT utilizing the model designed above has been illustrated. * The results of theoretical analysis, simulation, and sea trials are compared, confirming their effectiveness.