You might want to fill in the whole continent and then draw the rivers on a new layer. Then you can use this new layer as a mask on the land layer so that the ocean shows through and you still have rivers that you can play with...turn down the opacity, put on a stroke or drop shadow, etc. The major river violation that you have is that rivers start in mountains and end up in the ocean...they don't start in the ocean and end up somewhere else in the ocean. Simple fixes so it shouldn't take too long. Nice colors and not too shabby overall.