Thanks all,

...but sorted (kind-of): as Carnifex suggested, I'm working with shapes placed underneath the layer containing the coastal image. This is resulting in a series of concentric layers describing the depth of oceans (I'll have to find an elevation that I can saturate a background layer with before continuing with the deeper zones of water, but that's for later) placed underneath the coastal layer. At the moment I'm still tracing over work I did in photoshop, but once I'm done it'll get more imaginative (especially in regards to land formations - I won't post an image yet, because it's nowhere near finished). Once I've got a basic atlas I'll post some images.

Happy charting, fellows!