Select all objects (Ctrl A or Edit - select - all objects).
Right click - copy. This puts everything on the clipboard. Click outside the chart to kill all selections.
Now click on the land, hold down ctrl and click on your shoreline to highlight both. Hit delete. You are now left with just the island shorelines.
All your island shoreline polylines should be single closed polylines. This is very easy with the close polyline command, which can be applied if you select all the islands by dragging a selection box across the screen, then right click - close polylines.
Next select everything (ctrl-a), hold the cursor over one of the highlighted lines, right click and hit modify - kind - convert to polygon - Navico Land.
Now Ctrl-V or paste - and the islands are complete.
Next put in the chart background, it doesnt have to be exact as we will trim everything later.
Make sure to set the zoom levels for your islands to the same as that of the rest of the land.