Nice!

The base tiles look good!

My only feedback would be that the transition tiles could use a little work.

The grass to water transitions work quite well.   The mud bank at the top with the grass over hanging on the bottom and sides creates a nice bit of faux perspective.  I also like that on these, the grass seems to overlap more on the bottom than on the sides, again this aides the faux pespective of it.

The grass to dirt transition looks ok also, could maybe use a slightly more exaggerated shadowing along the bottom.  Well, it seems to have a nice fark shadow in the grass to dirt transition set (first set) but much less shadow in the dirt to grass set (second set).  I think it could also benefit from going a little thinner on the grass overlap along the sides.   Again, this seems to be the case in the grass to dirt but not dirt to grass set.

The dirt to water transition needs the most love.   The large overlap area and the strong shadow conspire to make the water look like it is above the dirt.  This is true in both the dirt to water and water to dirt transition set.  I would consider going with a hard edge along all sides for this one.  It can be bumpy/jaggy, but I would try to avoid heavy dithering here.  It works for the grass because it suggest little blades of grass but it doesn't work as well for water/dirt transition since both substances are generally more solid/smooth along the edges.  Similar to the grass/water transition, I would go with thicker shadow along the bottom, slight shadow along sides and no shadows (possibly a thin bright) along the top.

Well, that's just my two cents, hope it helps and thanks for sharing!