text: I think that's an excellent idea. We have a similar problem in Peterborough here: 
  https://osm.org/go/eu7wrWvZ--?layers=N&m=
  
  Strikes me that <ele=-0.1> is pretty fine-grained. Could we instead have a tag "floods_at=*", and could then predict "passable with difficulty", "impassable", with definitions for each of those? Bc it may be that absolute height of water is only one factor (eg, flow patterns)