Does it make sense to create relations for very large objects - say: the Mediterranean Sea?

They are already very large relations in OSM. For instance, countries boundaries (Germany, France but also probably Australia, USA, China, etc) or inter-states routes (highways/freeways). So, the question is not if it make sense but if you need them. You just have to know that very large objects imply very large relations which are not easy to create and maintain. For this reason, some relations are collecting sub-relations (for instance, the french boundary is composed of sub-relations containing the boundaries of each adjacent country). The OSM API is allowing this but then, the applications consuming OSM data (like renderers, routing apps, statistic tools, etc) have to correctly handle such special relations (having other relations as members) which is not the case today.
They are also very large objects where the outlines are not well defined (valleys, massif). You can find some proposals on the wiki about such things like http://wiki.openstreetmap.org/wiki/Proposed_features/Valley, http://wiki.openstreetmap.org/wiki/Proposed_features/Massif or http://wiki.openstreetmap.org/wiki/Proposed_features/Sea using nodes (so it's more placing a label) or even this relation proposal : http://wiki.openstreetmap.org/wiki/Relations/Proposed/Region but I don't know it's widely used today.