With the advance and wide applicability of the Internet of Underwater Things, smart things are deployed under the water and form the underwater wireless sensor networks, for the exploration of vast ocean volume. Generally, a sub-region, rather than the whole network region, may be interested by a certain application in a certain time duration. To address this challenge, a sub-region query processing mechanism is proposed. Specifically, a routing tree is constructed where a sensor node corresponds to a tree node. A sensor node acknowledges the fact that whether it lies within the sub-region of interest or not through the localisation techniques. Sensory data are routed to the sink node leveraging the parent-child relation of sensor nodes specified in the routing tree. Experimental evaluation shows that our technique is more energy efficient when the network topology is relatively steady.