A simple, low-cost and robust for the problem of multipath in underwater acoustic communication in deep water, is presented. Initially, a delay profile and noise level characterization, in offshore oil exploitation environments is done, with the objective of defining the communication channel of interest. The modulation techique, considered adequate by means of its simplicity and robustness is FSK. The non adequacy of an non-coherent FSK-2 system, without any contermeasure againt multipath is showed in experimental results. Two possible solutions to the problem of the demodulation of FSK-2 signals with multipath are presented. One very simple, based on the insertion of guard times between the bits, sufficient to the vanishing of the multipaths, that have been implemented and tested. The other, more elaborated, based in the estimation of the channel parameters, with the EM algorithm, and the analysis of some particularities of the problem, for the case of three rays. The second solution is analyzed and simulated in a computer. The results for both solutions are analyzed.