text: You can get the extent of the 'official' coastline from OS Opendata. Above that I'd draw the banks as a riverbank polygon. I'd extend the line of the way that is the river just over the edge of the coastline, so it overlaps the 'sea' slightly. 
  
  This is my idea, no more. It's how I treated the Humber estuary, though the coastline extends much farther inland than I imagined there.
  
  HTH 