Intersymbol interference (ISI) cancellation over frequency-selective underwater acoustic channel (UAC) is usually performed efficiently using iterative equalization. To this aim, an interleaver is required between the error correction scheme and the modulation scheme, and the longer the interleaver block length the better the performance. However, in the case of time-varying UAC, the length of the interleaver should not be longer than the channel coherence time, thus limiting the performance. Hence, for time-varying UAC with a low coherence time, small interleaver block length have to be used. In this paper, we propose a small block length coding scheme and its associate joint equalization and decoding algorithm based on Spinal codes. We show its efficiency over static large delay spread channels and static UAC: more precisely, simulation results show that the proposed scheme achieves a higher bit error rate than the Turbo equalization scheme with the same block length, especially for high spectral efficiency modulations. Simulations of proposed scheme over UAC obtained from underwater sea experiments are also presented which show the promising performances.