To make a river delta you could just make the brush a bit wider at the end to make a triangular shape connected to the river. You could also draw a few smaller streams splaying out into the bay. Don't make a lot, and make sure they are very close to the beach.

Shallow water typically has a lighter color than the deeper ocean/sea. This is pretty easy if you have layers and channels for your land/sea boundary. If you have no idea what that means, don't worry, just keep trying some more tutorials on the site I went through about two dozen (including this one) before I made my first map, they are generally very helpful.

As for general pointers/advice: ease up on the mountains, put them in clusters closer together, not spread uniformly across the whole map. Try adding in some more asymetry with the land/water. Save your image as a jpg instead of a pdf