Underwater Wireless Sensor Networks (UWSNs) are essential for a wide range of applications, including environmental monitoring, offshore exploration, and underwater surveillance. Due to the different challenges in the deployment of UWSNs, such as limited bandwidth, high propagation delays, and high energy costs etc. the performance evaluation of routing protocol becomes very important. The Vector-Based Forwarding (VBF) routing protocol is analyzed for its suitability for the underwater medium. In this study we examined the impact of varying packet sizes and node densities on two key performance metrics: average end-to-end delay and energy consumption in Multihop UWSNs. Simulations are conducted using NS2.3 integrated with Aqua-Sim, which accurately models underwater acoustic communication characteristics. The analysis shows how packet size and node density impact the energy efficiency and delay in a network. The results help to select an optimal packet size for a given network density, balancing delay and energy consumption according to specific application requirements. This study contributes to the design of more efficient and application aware UWSNs, particularly in scenarios where energy conservation and timely data delivery are critical with the consideration of packet sizes.