The presence of water in the mantle wedge overlying a subduction zone is expected to significantly enhance electrical conductivity, raising it above that of the cold subducting slab, or surrounding regions of dehydrated mantle. This suggests that magnetotelluric (MT) transects across subduction systems, measuring regional electrical conductivity structure, might be able to indirectly trace the pathways of water migration into the mantle. For ocean-continent subduction, it is logistically simpler to collect MT transects on the continental side of the system. However, we show that such land data are relatively insensitive to details of the electrical connections between the ocean and mantle. In contrast, seafloor measurements on the landward and seaward side of the trench are very sensitive to these electrical connections, and are essential to understanding the electrical structure of the entire subduction system. In particular, the conductivity structure of the hydrated mantle wedge overlying the slab can only be studied using offshore MT data. We demonstrate this result using a model of an ocean-continent system, although our results can be generalised to other subduction geometries.