Do you have all of your lineart on a single layer? If so, that is one thing I would HIGHLY suggest from a workflow perspective is to seperate everything as long as your machine has the ram to handle the image.

In your case, I would have, at a MINIMUM, layers for:
  • land outline
  • mountains line art
  • waves
  • forests


Then, anytime you make a selection for some reason, I almost always create a channel and/or a layer mask based upon such(depending on what I expect to do). The problem with putting it all on a single layer is that if you need to put a small blur on the outline of the island(for example), you then put that blur on all of the other line art which you don't want to do for the mountains and forest shapes(most likely anyway).

You have much of it already done, so you don't have to "redo" it all from scratch, what I would do is make use of some selections to create layer masks and the duplicate your line art later several times, changing the layer mask as needed. For example, your waves layer should be masked to the outermost shape of the island because you will never have waves INSIDE your island shape itself. Trust me, this keeps you from "accidents" that you don't see for days or weeks if you learn to use layer masks liberally(experience talking here).