Can I suggest that you also download 'Gimp' (which is also free and a raster image editor like photoshop). The Gimp will do many of the things that Inkscape cannot (texture creation, overlay blends etc). Also, What you might want to do then is to look at RobA's tutorial on making not so random coastlines in gimp (linked). This will allow to make a coastline which looks authentic but which will still follow the general direction that you want it to.
You can then load the resulting image into inkscape and run a 'convert to vector' on it (don't know what it's called in inkscape). This will give you a more 'real' looking coastline with lots of wiggly bits, small islands etc. Of course you can do this by hand in inkscape (much easier if you have a tablet and pen) but if you do not have a tablet and pen, then I suggest Rob's method since trying to draw a good coastline with a mouse is pretty time consuming!