Understanding the mechanical demands of an exercise on the musculoskeletal system is crucial to prescribe effective training or therapeutic interventions. Yet, that knowledge is currently limited in water, mostly because of the difficulty in evaluating external resistance. Here I reconcile recent advances in 3D markerless pose and mesh estimation, biomechanical simulations, and hydrodynamic modeling, to predict lower limb mechanical loading during aquatic exercises. Simulations are driven exclusively from a single video. Fluid forces were estimated within 12.5 +/- 4.1% of the peak forces determined through computational fluid dynamics analyses, at a speed three orders of magnitude greater. In silico hip and knee resultant joint forces agreed reasonably well with in vivo instrumented implant recordings (R-2 = 0.74) downloaded from the OrthoLoad database, both in magnitude (RMSE = 251 +/- 125 N) and direction (cosine similarity = 0.92 +/- 0.09). Hip flexors, glutes, adductors, and hamstrings were the main contributors to hip joint compressive forces (40.4 +/- 12.7%, 25.6 +/- 9.7%, 14.2 +/- 4.8%, 13.0 +/- 8.2%, respectively), while knee compressive forces were mostly produced by the gastrocnemius (39.1 +/- 15.9%) and vasti (29.4 +/- 13.7%). Unlike dry-land locomotion, non-hip-and non-knee-spanning muscles provided little to no offloading effect via dynamic coupling. This noninvasive method has the potential to standardize the reporting of exercise intensity, inform the design of rehabilitation protocols and improve their reproducibility.