Purpose: To compare ventilatory and cardiorespiratory responses between the COSMED AquaTrainer coupled with the K4b2 and K5 wearable metabolic systems in breath-by-breath mode over a wide range of swimming speeds. Methods: Seventeen well-trained master swimmers performed 2 front-crawl 7 × 200-m incremental intermittent protocols (increments of 0.05 m·s−1 and 30-s rest intervals, with a visual pacer) with AquaTrainer coupled with either K4b2 or K5. Results: Post hoc tests showed that swimming speed was similar (mean diff.: −0.01 to 0.01 m·s−1; P = .73–.97), repeatable (intraclass correlation coefficient: .88–.99; P < .001), highly accurate, and precise (agreement; bias: −0.01 to 0.01 m·s−1; limits: −0.1 to 0.1 m·s−1) between all conditions. Ventilatory and cardiorespiratory responses were highly comparable between all conditions, despite a “small” effect size for fraction of expired carbon dioxide at the sixth 200-m step (0.5%;