Locomotion in fluid environments, such as air and water, is ubiquitous in natural and engineered systems. In active locomotion, a system uses its own or locally available energy to move, doing work on the surrounding fluid. In contrast, passive locomotion occurs when external forces from the environment set the system in motion. Both forms of locomotion in fluid environments involve complex fluid-structure interactions, offering intriguing dynamics and opportunities for advantages in force generation and performance efficiency. In this thesis, we explore these aspects for three different systems - one biological and two engineered - from a fluid physics perspective. In the first work, we study the drinking flight maneuver of bats to unravel the systematic differences in kinematics and aerodynamics relative to its regular straight flight maneuver, using flight experiments and mathematical modeling. Our results reveal a consistent change in the flapping amplitude and flapping frequency across two species. Furthermore, an error minimization technique, for understanding aerodynamics, revealed a lift enhancement in drinking flight. In the second work, we investigate the interaction of a synthetic active swimmer, which self-propels by generating local surface tension gradients at the air-water interface, with passive particles in its environment, under one-dimensional confinement. Using videography experiments, we reveal a transition in swimmer dynamics with increasing particle concentration. Moreover, our experiments also indicate a cargo transport potential of the swimmer under confinement, which is modelled using an analytical approach. In the third work, we examine a passively flapping paper airplane using free flight experiments and mathematical modeling. High speed videography of flight experiments revealed a stable flapping behavior accompanied by an in-phase oscillatory pitching. Parametric study by varying wing size and paper thickness unveiled a flapping frequency increase with smaller wing area and a transition to gliding. Finally, we present a reduced-order mathematical model which reproduces the dominant dynamics observed in experiments.