GiMP - delete areas outside an image

GIMP mostly does exactly the opposite of what I want in a few steps, or takes many many steps to do what I want, or the tutorials leave out one critical step that costs my weekend. In other words, I hate GIMP, but it's free. Take the following image of an old duct-taped surfboard that I found online.

Suppose I wanted to delete the background and keep the foreground surfboard. You'd think, OK, 3 minutes,3 steps: take the magic wand/select, add transparency, then bucket fill the selected parts. Two problems: 1) the surfboard has black fins, so fuzzy select goes into the surfboard, 2) bucket fill fills a selected area, not areas outside a selected areas. So now I have to do this differently, and with a shitload of steps.

Tutorials (10 minutes):

1. Do all the selection steps, click click path nodes, then hit edit and bend them. Then Select by Path
2. Add alpha transparency layer
3. Delete key

Your image will be deleted and your background will be left, just as if you used the bucket fill tool.

2 hours later, the Real (10 minutes):

1. Do all the selection steps, click click path nodes, then hit edit and bend them. Then Select by Path
2. Select-->Invert. This swaps what's selected with what's not selected or, say, the background with the foreground.
3. Add alpha transparency layer
4. Delete key