You have a strong contrast between the land (bright) and the sea. Not a real problem, but it's really drawing your attention to the land, imo.
Your coastline looks a bit pixellated, maybe a "blur" filter could change that. I tend to not use pure black/white, and I think your coastline and ripples are too b/w. You can just play with opacity to correct that anyway, just as you mentionned.
Don't hesitate to play with opacity/layer effects. Sometimes, you just turn something good into something really great just by playing with your different layers settings .
Keep up the tests & tries, I'm confident you'll come with an excellent map .