Well, there is.
It's not pretty (quite literally) but it works.
Of course, I would prefer proper 2D "coast" graphics that would let me identify beach / cliff / land-submerged borders at a glance.
Some kind of... line, I guess... that goes from the center of one coast square to the next. Colour and shape of the line would signify the type of coast.
Right now the coastline (literally a thin black line) seems to be derived from the 3D graphics. While these do work in 3D (because you see that they are cliffs) this line is simply not useful in 2D where the visible cliff graphic is... not there. As a result, many "cliff" tiles look like grassy plains on the 3D map because they are 95% "ground".