1.) Use Random Item Placer, don't have an Website atm for you. Try to search with google
2.) Use Worldforge (downloadable here) and "play" something with the Z-Altitude. Take a look at the guides. There is no real existing guide for having "the perfect looking coast lines". It always depends how you have painted your map. If your water is -5 and your gras (or anything else) starts at 0, try to lower the z altitude (from the gras) a little bit at this area where it looks "ugly". Sometimes it helps a little bit. As i already said, there is no solution for the "perfect coastline". The only hint i can give you is to avoid long distances of straight coasts. Make better some small bumbs with the width of 3 pixels (or more). This will give you better results and it will looks more natural, too
bad example :
http://cyranor.online.de/bad.gif
good/better example :
http://cyranor.online.de/good.gif
Sorry for my bad english, i hope someone can understand it