Very nice; I haven't even tried Inkscape yet, but it looks like an excellent program. You're right about the grid lay-over being helpful; maybe take a look HERE for Cartesian Grids and HERE for regular grids (same page, different sections). Again, I don't know inkscape, so I have no idea how to actually apply/use those "effects".

The landmasses look good, and the coasts look realistic. I especially like the more jagged coasts breaking up/into the "smoother" coasts (compared to the "jagged" coasts).