You can also then see, and avoid incorporating, any untidy bits you leave behind when you made previous selections. It also eliminates any antialias ghosting problems. Now is a good time to save your work. For this reason, if you plan ahead you can leave the most fiddly lot of selections to last to minimize your heartburn. Follow this procedure for each group of selections in your graphic.
Flood fill the new background layer with white just to make sure it is clean.
See below for information about cutouts. Cutouts on a Selection on a Transparent Layer From File Preferences, customize your toolbar. If a section is NOT floating when you perform a cutout on a transparent layer, you will cut out the colour of the selection and be left with only the shadow you applied. If you have the icons on your toolbar you can see easily what state a selection is in. If the small yacht is highlighted, you need to click it to float the selection before applying a cutout.