Being a layer mask evangilist, I would suggest a layer mask instead. There are two ways, I would suggest the first:
- Very bottom layer, fill with the darker blue color
- Second layer from bottom, fill with lighter blue
- Make a selection of all the land(or all the "sea")... I typically turn off all of the layers except the outline one.
- (this step is optional, but highly recommended so you have a "copy" of your layer mask before the next step, so if you want to get back to a base line, you can do so much easier)Select Selection->Selection to Channel(I am doing this from memory, so the exact wording is likely off a bit). This creates a "Channel" on the Channel tab.. make sure you give it a name that makes sense to your context such as "Land Outline". IMPORTANT!!!! depending on if you selected the land or the sea in the previous step, you may need to do Selection->Invert... you want to make sure that your Channel has BLACK for the land and white for the sea when you create the channel(and you can even do it twice and create two channels with each and inverse of the other if you want)
- Right click on the second layer from the bottom and select Add Layer Mask, adding the newly created channel(again, making sure that you pick the one with the white part where the sea is)
- Selection->None(make sure nothing is selected in the main image window!!!)
- On the layers dialog tab/window/whateveryouwannacallit, click the layer mask that you just created(not just that layer "row", but the mask which will show as a B/W box to the right of the blue water color this layer is filled with.
- use Filters->Gaussian Blur to suite
- as needed, you could also pick a grunge style brush with white color selected and the airbrush tool to paint a bit more of the "sea" with white which will show more of the darker blue below.
- Make sure you click OFF of the layer mask before going any further
- Put all the other layers above these two.
Alternate:
- Very bottom layer, fill with the darker blue color
- Second layer from bottom, fill with lighter blue
- Add layer mask to the second bottom layer filled with black
- Select an appropriate brush size, preferably a nice fuzzy brush
- Select the airbrush tool
- On the Second layer from the bottom in the Layers dialog window, click OFF of the main layer part and click ONTO the layer mask itself.
- lightly "paint" the white onto the black layer mask. Everywhere the white is on the layer mask, the layer below will show through. Make sure that you overlap the mainland by a decent margin
- right click and select "Mask to Selection"(this step is optional, but highly recommended so you have a "copy" of your layer mask before the next step, so if you want to get back to a base line, you can do so much easier)... this creates a "Channel" on the Channel tab.. make sure you give it a name that makes sense to your context such as "Land Outline"
- While still having the layer mask selected, use Filters->Gaussian Blur to suite