You haven't said in what context you want to interpret these terms but a couple of things come to mind:
Safety equipment equipment, flares etc.Inshore - up to 3nm, Coastal upto 7nm and Offshore over 7nm. Also trying looking RYA Boat Safety handbook (C8).
Recreational Craft Directive has a classification (a, B, C, D) based on wind and wave conditions rather than distance. Offshore (b) wind force 8 and significant wave heights to 4m, Inshore (c) force 6 and 2m, Sheltered (d) such as force 4 and 0.3m (occasional 0.5). Note that C and D can be inland depending on the expansive of water and local conditions. Classsification A is for Ocean going vessels.