This paper presents some results on adaptive minimum mean-square error (MMSE) turbo equalization obtained from underwater experiments in the Atlantic ocean. The performance gain is evaluated as a function of the number of hydrophones at the receiver side. Single-carrier transmission with high-order modulations is considered, at a coded bit rate as high as 24 kb/s on the underwater acoustic channel. The all-digital receiver performs timing recovery, equalization, interleaving and channel decoding.