Most knowledge of polar corrosion comes from atmospheric exposure trials; in situ immersion data for Antarctic infrastructure are nearly absent, and immersion in glacier meltwater has, to our knowledge, not been previously reported. During the XXIX Ecuadorian Antarctic Expedition, ASTM A36 carbon steel coupons were immersed for up to three weeks at Pedro Vicente Maldonado Station (Greenwich Island) in two chemically contrasting waters, namely coastal seawater (34.3 PSU, 48.5 mS cm⁻¹) and glacier meltwater (0.04 PSU, 79.3 µS cm⁻¹; conductivities measured at 18–20 °C), which have an approximately 600-fold conductivity difference. Corrosion rates were determined gravimetrically (ASTM G1/G31) and cross-checked with open-circuit potential, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP). Gravimetric rates showed no statistically significant difference between environments (47 ± 4 µm yr⁻¹ in seawater versus 51 ± 9 µm yr⁻¹ in glacier meltwater; Welch’s t-test, p = 0.28), directly contradicting the expectation that higher conductivity and chloride content would accelerate attack. Raw LPR suggested an apparent five- to ten-fold higher polarization resistance in glacier meltwater, but EIS showed that this was almost entirely uncompensated solution resistance (median Ru·A ≈ 14 kΩ cm²), whereas the EIS-derived polarization resistance was of the same order in both waters (median Rp·A ≈ 2.4 kΩ cm² in seawater and 1.3 kΩ cm² in glacier meltwater). PDP consequently underestimated the glacier corrosion rate (≈7 µm yr⁻¹) due to IR-drop distortion of the Tafel slopes in the low-conductivity electrolyte. Scanning electron microscopy and energy-dispersive X-ray spectroscopy showed Fe- and O-rich corrosion products in both waters, but seawater forms a dense, continuous, and chloride- and sodium-bearing crust, while glacier meltwater forms sparse, isolated florets incorporating silicon from glacial rock flour and no detectable chlorine. The results show that, over a single Antarctic summer, glacier meltwater is not a benign environment for carbon steel and that gravimetric measurement, rather than uncorrected electrochemical techniques, is required to obtain reliable corrosion rates in low-conductivity polar waters.