To extend the structure life of offshore steel structures, corroded or damaged structures can be repaired using adhesively bonded composite reinforcements. As polymers are sensitive to moisture, the durability of such solutions in the offshore environment needs to be studied to be able to propose adapted design methodologies and safety coefficients. Water diffusion in homogenous materials benefits of rather good knowledge. Yet, when a material is in contact with another one, sorption kinetics are modified and more complex to analyze. Actually, a transition zone, called interphase, is created and can be a weak point as far as diffusion is concerned. The presented study aimed at studying the effect of those interphases in the context of adhesively bonded Fiber-Reinforced Polymer (FRP) patches developed for offshore steel structures. It relied on an extensive experimental campaign on immersed samples, monitoring diffusion rate using gravimetry. Two different kinds of samples have been analyzed: bulk disk samples, and bi-material samples containing interphases. Two different type of interphases were investigated: steel to polymer interface and polymer to polymer interface. The obtained results highlighted the influence on the interphases on diffusion rates and maximum mass water content after ageing.