Swarming is a multicellular mode of motility common to flagellated bacteria species which enables coordinated rapid surface translocation, expansion, and colonization. Swarming typically follows bacterial cells changing phenotype from planktonic to elongated and hyper-flagellated states and is triggered by the presence of soft elastic, and permeable surfaces. These motile and multi-cellular bacteria colonies display characteristics uniquely representative of wet active matter systems, including intense fluctuating vortices, self-emergent long-ranged velocity fields, and persistent flocks. In this dissertation, I report on dense and semi-dense suspensions of the canonical swarming species, Serratia marcescens under various conditions when hydrodynamic interactions between bacterial cells is important. First, I present experimental and computational studies of dense swarm front interacting with and moving through domains of immotile bacteria that resist motion. I show that the active-passive swarm interface in this system has unique morphological features that are critically dependent on the relative importance of hydrodynamic interactions. The swarm region adjacent to the boundary develops spatially periodic and transient vortices of alternating sense that continuously convect immotile cells away from the interface, allowing for the active swarm to move into that territory. Additionally, the roughness of the evolving interface exhibits partial self-similar features that I compare with classical, continuum interface models. Motivated by the important role played by hydrodynamics in dense swarms, I next present experimental and computational results relevant to dilute systems such as the pre-swarming state or bacterial cells in the vicinity of a free interface. Here the object is to focus on the role of cell length and hydrodynamics and identify their individual and synergistic impacts on emergent collective motion. To complement experimental data, and circumvent the difficulty in devising experiments that decouple these effects, I explore the role of cell aspect ratio, cell-cell interactions, and hydrodynamics using a minimal agent-based model that treats the swarm as a suspension of self-propelled active rods (cells) moving in a plane. These in silico swarmers have tunable cell size, aspect ratio, cell-cell interactions, and fluid mediated hydrodynamic interactions that allow for exploration of these effects independently and in combination. I find that an increase in aspect ratio enhances overall cluster size and cluster persistence time. Hydrodynamic effects have a mixed effect and may either stabilize emergent structural features or weaken them. Strong hydrodynamic interactions can destabilize large-scale structures, due to significant fluid and velocity gradients that depress and prevent persistent clustering. I conclude this dissertation with a chapter detailing the workflow to image, segment, and analyze immotile Serratia marcescens as they proliferate, elongate, and form motile multi-cellular forms ranging from small clusters to large correlated swarming domains. This workflow will aid significantly in future investigations into the physical mechanisms that initiate the swarming process, as well help quantity these living active polar nematic systems.