A melt of linear diblock copolymers (A(n)B(m)) can form a diverse range of microphase separated structures. The detailed morphology of the microstructure depends on the length of the polymer blocks A(n) and B-m and their mutual solubility. In this paper, the role of hydrodynamic forces in microphase formation is studied. The microphase separation of block copolymer melts is simulated using two continuum methods: dissipative particle dynamics (DPD) and Brownian dynamics (BD). Although both methods produce the correct equilibrium distribution of polymer chains, the BD simulation does not include hydrodynamic interactions, whereas the DPD method correctly simulates the (compressible) Navier Stokes behavior of the melt. To quantify the mesophase structure, we introduce a new order parameter that goes beyond the usual local segregation parameter and is sensitive to the morphology of the system. In the DPD simulation, a melt of asymmetric block copolymers rapidly evolves towards the hexagonal structure that is predicted by mean-field theory, and that is observed in experiments. In contrast, the BD simulation remains in a metastable state consisting of interconnected tubes, and fails to reach equilibrium on a reasonable time scale. This demonstrates that the hydrodynamic forces play a critical part in the kinetics of microphase separation into the hexagonal phase. For symmetric block copolymers, hydrodynamics appears not to be crucial for the evolution. Consequently, the lamellar phase forms an order of magnitude faster than the hexagonal phase does, and thus it would be reasonable to infer a higher viscosity for the hexagonal phase than for the lamellar phase. The simulations suggest that the underlying cause of this difference is that the hexagonal phase forms via a metastable gyroid-like structure, and therefore forms via a nucleation-and-growth mechanism, whereas the lamellar phase is formed via spinodal decomposition.