Abstract: In this paper, the underwater channel is modeled as Rice or Rayleigh channel. The formulas of symbol error probability P, bit error probability P' of FSK block code and bit error probability P of FSK convolutionary code are obtained. The contribution of the determined part of signal is found on the exponential terms of P, and the contribution of the random part of signal is found on the linear inverse proportional terms. When the determined part of signal is increased, P is increased fastly; when P equal to P', the frequency width required by a simple convolutional code is considerable smaller than the one required by block code. The convolutionary code applied to underwater communication is very significant.