The average bit error rate (ABER) performance of the decode-and-forward-based multiple-input multiple-output (MIMO) multi-hop underwater wireless optical communication (UWOC) systems has been investigated over the composite exponential-generalized gamma (EGG) fading channel with space-time block coding (STBC). The impacts of absorption, scattering, and the misalignment caused by spatial spreading in the sea, which are modeled by the beam spread function, as well as the turbulence-induced fading are all taken into account in this work. With the help of the moment generating function, Meijer’s-G function, and Gauss–Laguerre quadrature numerical methods, the approximate closed-form expressions of ABER for the UWOC system with binary phase shift keying are mathematically derived. Furthermore, numerical analyses are provided to investigate the effects of various hops, temperature gradients, air bubbles levels, STBC schemes, and turbulence severity on the performance of the multi-hop MIMO UWOC system. In addition, Reed–Solomon (RS) codes are adopted to improve the UWOC system performance. Results reveal that the ABER performance of this multi-hop UWOC system would degrade as the temperature gradients, hop numbers, and bubble levels (BLs) increases, which could be improved considerably by employing the STBC scheme. The ABER performance over the composite EGG distribution is mainly limited by the environmental parameters, whereas the fading can be significantly improved by RS codes. The ABERs of EGG distribution are compared with exponential-gamma and exponential-lognormal distributions in oceanic turbulence under different temperature gradients and BLs, respectively. These analytical results are also verified by Monte Carlo simulations. This work is beneficial for the design of UWOC system.