The minimum-length scheduling problem has recently been studied for wireless networks with a realistic physical interference model. In these approaches, the propagation delay is ignored due to the relatively fast speed of RF waves compared to the transmission duration. However, signals in underwater acoustic networks travel at a much slower speed, so the propagation cannot be ignored in determining the transmission schedule. In this work, we highlight the difficulty in determining the minimum-length schedule for networks with non-negligible propagation delays, and we identify some special cases in which the approach without propagation delays can be applied, in some cases yielding an optimal schedule.