Yep, you might be past those regional tutes now ...

1. you can blend the sea with the land by blurring your layer mask. This will smudge the hard edge and help a smooth transition between the land and the sea. Is this what you mean when you say 'feathering'? If not, can you post your current version and say what it is that you don't like?

2. White. Yep, it's a problem with extreme values. If you have white or black, soft light layers won't touch them. If you have a light region in your lower layer, then the light regions of the soft light layer will have more effect than the dark regions. If you have a dark region in the lower layer, the dark regions have more of an effect than the light regions. If it's white, then the light regions lighten it - but it's white so you don't see anything - and the dark regions have no effect.

There are a couple of solutions. First, you can use a darker shade than pure white. Try something like an 80% grey - perhaps tint it a little blue. The emboss layers should lighten up the highlights to almost pure white and the shadows will still show.

The alternative is to duplicate your emboss layer and set it's blend mode to multiply and a low opacity, say 20%. That will lock in shadows independent of what the lower layers look like.

3. With the smudge - make sure you use a soft brush so that you don't get hard edges on it. Post what you've got and why you don't like it and I'm sure someone here will be able to offer a solution.