Developing ocean-going unmanned robotic systems has been a focus for the marine research community for many years. Compared with earlier manned submersibles, the current state-of-the-art autonomous underwater vehicles (AUVs), tethered remotely operated vehicles (ROVs) and unmanned surface vehicles (USVs) augmented with the advancement in the sensor technology offer dramatic improvements in safety, cost, and efficiency, especially for deep water sensing operations. However, coastal zones such as estuaries and river deltas that are highly productive habitats supporting a variety of fish and wildlife may be challenging for the current suite of platforms. The complex geographical features in these regions, such as land barriers, icebergs and tidal currents, may hinder the movements of the aforementioned platforms. For this reason, a complementary sensing paradigm that employs waterproof unmanned aerial vehicles (UAVs) integrated with underwater sensors is proposed. The implementation of such concept -the Hybrid Aerial Underwater Robotic System (HAUCS) is presented. The development of one HAUCS platform, the coaxial waterproof drone, is discussed.