Swimming is a critical task for helicopter rescue paramedics and aerobic capacity is assessed in this occupation to determine job suitability. We evaluated one treadmill-based and one pool-based assessment of maximal aerobic capacity (V̇O2peak) in 14 helicopter rescue paramedics. There was a small absolute difference (p = 0.11, d = 0.46) between V̇O2peak in the swim (45.5 ± 7.8 ml.kg-1.min-1) compared to the run (48.5 ± 5.5 ml.kg-1.min-1), with a moderate relationship noted (r = 0.74, 95% CI [0.35-, 0.91], p = 0.0023). Whilst not interchangeable, run V̇O2peak was a predictor of swim V̇O2peak. Maximal blood lactate was similar (p = 0.93) in swim (13.4 ± 3.8 mmol.L-1) and run (12.2 ± 3.0 mmol.L-1), and maximal heart rate 13% lower (p < 0.0001) in the swim (162 ± 11 bpm) versus the run (186 ± 10 bpm). To estimate swimming V̇O2peak in paramedics a treadmill test is sufficient but does not replace assessment of swimming proficiency.PRACTITIONER SUMMARY: We developed a swim protocol to assess maximal aerobic capacity in helicopter rescue paramedics. Compared to a treadmill-based test, our swim protocol generated 20% lower submaximal V̇O2 and 6% lower V̇O2peak. Although not interchangeable, a treadmill V̇O2peak test is indicative of maximal aerobic capacity in rescue paramedics whilst swimming.