[Stiglr]

You could start by getting a nicely detailed elevation map and using that as a starting point. Using the Poly tool, you could describe the coasts of the various islands. Then the challenge would be how to extrude the elevations.

I dunno if AC3D can make use of actual geographic data; if it can, that'd make your job much easier. You could get the data from GTOPO, probably free.

Also, if this is for a game, does your game have a tool kit that allows you to create terrains? That might be a better approach than a 3D program.