Durable reinforced concrete is a fundamental requirement in a marine environment, but at the same time it must be sustainable, meaning its production emits the least amount of greenhouse gases. Hence, the importance of achieving optimal proportioning methods. This paper presents and discusses the electrochemical differences in the passivity state of reinforced concrete specimens designed using two proportioning methods: M1 prioritizes the ultimate strength of the element, and M2 focuses on sustainability through optimized aggregate arrangement and reduced cement content. Small beams (150 mm × 150 mm × 300 mm) with varying cover thickness (15 mm, 20 mm, and 30 mm), with two water/cement ratios (0.45 and 0.65), all utilizing Portland composite cement (PCC 30R), were exposed in a tropical marine environment 50 m from the seashore in the north of the Yucatan Peninsula for 700 days (passive state). Corrosion rate, corrosion potential, resistivity, and internal conditions (relative humidity and temperature) were periodically measured. A key finding revealed that M2, despite its sustainable advantage, tends to depassivation before M1, at least during the two years of exposure and while in the passive state.