<p>Underwater wireless sensor networks (UWSNs) hold great potential for improving oceanographic research and resource exploration, especially in monitoring changing maritime environments. However, these applications face several challenges, such as limited bandwidth, high energy use, and constantly changing network topology, which affect data reliability and efficient transmission. Currently, UWSNs often run into problems like using up too many resources and losing packets, which ends up hurting their performance. To maintain performance in UWSNs, the transport layer plays an important role. It ensures reliable and efficient communication by addressing the unique challenges of UWSNs. It manages energy efficiency, and congestion control to the underwater environment’s constraints. These protocols also adapt to dynamic network conditions, ensuring end-to-end reliable data delivery and effective traffic management, which is crucial for maintaining communication integrity in underwater environments. In this paper, a new lightweight transport layer protocol namely TCP Euler, is proposed for UWSNs which is inspired by Euler’s totient function. This protocol employs number theory and dynamic window drop mechanisms to enhance reliable data delivery, reduce energy consumption, and minimize packet losses in unpredictable underwater environments. The adoption of TCP Euler in the research community can significantly benefit various applications, including environmental monitoring, underwater exploration, and disaster response, by improving the effectiveness and reliability of UWSNs.</p>