You won't get to the Mon & Brec by narrowboat, apart from the fact that it's not connected to the Severn, even if it were, that estuary there is serious water, even experienced boaters would think a few dozen times before trying it. You can get from Sharpness to the Bristol Avon, but that's another trip for the well equipped, experienced boater- with a pilot on board.

If Severn is a clue, it ought to be a trip round the Worcester and Birmingham, BCN, the Staffs and Worcester to Stourport, and complete via the Severn.