Recent studies investigated the hydrolysis of Magnesium (Mg) thin films, highlighting their potential use in biodegradable devices. Quantitatively monitoring the degradation rate and morphological changes of Mg through optical methods and light-delivery devices offers an innovative, contactless technique, independent of electrical wiring. This method is particularly useful for challenging sensing applications. This study introduces a set of strategies based on Mg optrodes where hydrolysis drives the real-time monitoring of biofluid penetration through thin-film encapsulations for bioelectronics. This enables a straightforward assessment of their long-term reliability, through a quantitative correlation between the water transmission rate (WTR) of the encapsulation and the Mg optical modifications. The optical response of the corroding Mg films deposited on glass substrates and multimode optical fibers tips, within the visible spectrum is characterized. Finally, it is demonstrated that nanopatterning of Mg films as plasmonic nanoantennas significantly enhances the sensitivity of the quantitative approach in the mid-Infrared spectrum through localized plasmon effects. This method achieves a WTR detection of 6.9 × 10−3 gm−2 day−1 in phosphate buffer solution (25 °C), with a theoretical lower detection limit of ≈10−5 gm−2 day−1. These findings pave the way for the development of a new class of nano-optical water-permeation sensors.