Fluid status (FS) is a diagnostic challenge in critically ill patients with COVID-19. Here, we compared parameters related to FS derived from cumulative fluid balance (CFB), bioelectrical impedance analysis (BIA) and venous congestion assessed by ultrasound (VExUS) to predict mortality. We retrospectively reviewed the medical records of individuals with severe pneumonia due to COVID-19 between July and November 2021 in a single center. Comorbidities, demographic, clinical and laboratory data as well as results from CFB, BIA and VExUS measurements were collected on admission and weekly afterwards for two consecutive evaluations. Seventy-nine patients were included, of which eighteen (14.2%) died. Abnormalities of FS were only identified by BIA. Extracellular water/total body water ratio (ECW/TBW) > 0.394 (overhydrated) by BIA was a good predictor of mortality (AUC = 0.78, 95% CI: 0.067–0.89). Mortality risk was higher in overhydrated patients (OR: 6.2, 95% CI: 1.2–32.6, p = 0.02) and in persistently overhydrated patients (OR: 9.57, 95% CI: 1.18–77.5, p = 0.03) even after adjustment to age, serum albumin and acute kidney injury (AKI) in stages 2–3. Time to death was shorter in overhydrated patients (HR: 2.82, 95% CI: 1.05–7.5, log-rank test p = 0.03). Abnormalities in FS associated with mortality were only identified by BIA in critically ill patients with COVID-19.