First Overworld in Photoshop

[Tancread]

Between the Spider Sea and the Bay of the Reaper I am not sure I would want to be a surfer in the Kingdom of Mothica. Nice Map!

[torstan]

I don't think you need to add any emphasis to your coastline. The colour change and nice heavy line make it stand out pretty well. If you still want to do something you can try one of the following:
1. Add an outer glow to the coastline using a drop-shadow from the land with the colour set to white and then set the drop-shadow layer to overlay or soft light.
2. add water ripples to the sea along the edge of the coastline. These can either be in black, or white set to soft light.
3. Add wood-cut like horizontal lines coming away from the coast-line. There's a really well known tutorial on the effect that I can't put my finger on right now.

Hope that helps.