Today's oceans contain 26-66 million tons of waste, with approximately 94% located on the seafloor. So far, collection efforts have focused mostly on surface waste, with only a few local efforts to gather underwater waste, always using human divers. No solution exists that exploits autonomous robots for underwater litter collection; the SeaClear project will develop the first. We will create a mixed team of Unmanned Underwater, Surface, and Aerial Vehicles -- UUV, USV, UAV -- to find and collect litter from the seabed and from the water column, focusing on coastal areas since that is where waste inflow concentrates. The UAV and one or several inspection UUVs map the litter, aiming to establish correlations between surface and underwater litter. One or multiple collection UUVs then classify and collect litter, using a combined suction-gripper manipulator for both small and large waste. The UUVs are tethered to offload power and computation to the USV. Our objective is to operate the robots autonomously, without remote human intervention, and to that end we plan novel developments in debris mapping, classification, and robot control. When fully operational, the SeaClear system aims to detect and classify underwater litter with 80% success rate, and collect it with a 90% success rate; all this at 70% reduced cost compared to divers. We will demonstrate these features in two case studies: one in port cleaning (with end-user Hamburg Port Authority), and the other in a touristic area (Dubrovnik -- with end-user DUNEA). Besides the two end-users, the consortium includes an SME supplying proven hardware for our platform; an experienced marine system integrator; and four academic institutions with complementary expertise in underwater and aerial robotics, sensing, mapping, and control. The feasibility of SeaClear is completed by an exploitation and dissemination strategy that actively involves scientists, public and industry stakeholders, and Digital Innovation Hubs.