Though tremendous progress has been made for land-based radio frequency (RF) wireless communications, research in underwater acoustic (UWA) communications is still at a primitive stage. On the other hand, the ocean covers 71% of our planet, and is becoming the center stage as the resource shortage drives us to ﾓthe dawn of deep ocean mining.ﾔIn this area, our research focus is on the horizontal underwater communications where the channel conditions are very challenging with extensive multipath and Doppler-induced variation. Our group has developed several pioneering approaches. Some were recently tested in sea trials carried out by Woods Hole Oceanographic Institution (WHOI) at the Narragansett Bay, Rhode Island and in Elba, Italy. The results are very encouraging as our robust designs achieve superb performance even when existing alternatives fail, especially in turbulent sea. In this presentation, we will also introduce some of our designs and results on UWA.