We are currently developing methods for underwater communication that can form a basis for connecting underwater systems, and allowing them to become accessible nodes in a network centric defence. Here we study methods for both high data rate (> 1 kbps) acoustic transmissions and low data rate (1-100 bps). In the former case we use bandwidth efficient modulation, i.e. methods that allow more than one bit per Hz bandwidth to be transferred, of the type M-QAM or M-PSK. In the latter case our ambition is to study methods that reduce the probability for the transmitted signals to be detected or intercepted, i.e. covert acoustic communication. One promising way of achieving this is to use spread spectrum modulation combined with Turbo coding. Here we report on our initial work along this direction. In a previous study we reported on results from transmitting 8000 bps at a range of 14 km east of Gotland using decision feedback equalization (DFE) of short 4-QAM signals. The DFE is a standard method for combating the multipath distortion, which is characteristic of the channel. A single hydrophone was used as a receiver. Here we extend our results to larger distances and to process several hydrophones simultaneously, i.e. we study a multichannel DFE. In this way we manage to decode 4000 bps at a range of 38 km. The quality of the results are improved by using several hydrophones. We also introduce an alternative way of dealing with the multipath distortion through the use of orthogonal frequency division multiplexing (OFDM). We successfully decoded 5200 bps at a range of 4 km, using data from an experiment in Långnabba, a test range in the Swedish archipelago. We conclude the study by demonstrating that acoustic communication can offer performance in terms of range and data rate which sometimes is comparable to the standard HF-radio.