This paper addresses the issue of energy efficiency and the optimal data packet size/length in underwater wireless network communications in the context of effective and efficient data transmission. In light of the limited battery power of underwater sensor nodes, energy efficiency is investigated in relation to the optimization of data packet length to meet the low power constraints of the shallow water acoustic channel. The main contribution of this project is a data packet size optimization look-up table or graph of which viability is proven through virtual simulations using ns2 simulator and its MIRACLE packages on the Ubuntu platform. © 2011 IEEE.