The cell borders in Dragonborn seem to be about two feet apart, so trying to navmesh from the front door across a drawbridge and onto a boat crosses about six cells - I'm not even kidding!

Trying to draw the navmesh, it slams to a halt at the edge of the cell. How do I navmesh across one cell into the next cell?

Treat the navmesh in each cell as a separate entity. When connecting between two cells, ensure that the points on each side of the cell boundary line up with each other. Finalizing the navmesh will create the connection between the two cells.

Brilliant - I'll do that. Thank you so much