As a part of the first of three sea trials in the international collaboration "Undersea Surveillance and Communication" (USC), experiments with acoustic underwater communications were performed in November 2017 in the Stockholm archipelago. USC is a collaboration between FOI and the following United States organizations; NIWC- Naval Information Warfare Center; NRL - Navy Research Laboratory and NSWCCD - Naval Surfave Warfare Center Carderock Division. Signals using a large amount of modulation formats (different data rates), and several different center and symbol frequencies on the interval 3-35 kHz were transmitted. Both stationary and mobile transmitters were used in the sea trial, while all receivers were fixed. Communication ranges varied from very small to a maximum of approximately 5.4 km. Experiments where "line-of-sight" was lost between source and receiver were also conducted. The coherent modulation formats performed well in line-of-sight conditions, but broke down quickly when the direct path was lost. The non-coherent modulations have been introduced to address this situation. However, the results were almost as poor as for the coherent modulations. Further work is ongoing to improve the performance in later measurements.