If the two pictures are layers in the same Gimp image, it's easy because the selection applies to to all layers, so you can use one layer to create a car shaped selection, the activate the other layer, and Edit>Copy will copy the part of the other layer which is in the selection.
If you have two different images it's a bit more complicated because you have to transfer the selection from one image to the other.
If you uses paths to create the selection, with the two images open in Gimp, pick the path in the Paths list of the car image and drag it to the canvas of the sea image, and this will copy it across the images. Then in the sea image, you can adjust the shape (move, scale or else) of the path and then create the selection with Select>From path.
If you didn't use a path, then:
If you have two different images it's a bit more complicated because you have to transfer the selection from one image to the other.
If you uses paths to create the selection, with the two images open in Gimp, pick the path in the Paths list of the car image and drag it to the canvas of the sea image, and this will copy it across the images. Then in the sea image, you can adjust the shape (move, scale or else) of the path and then create the selection with Select>From path.
If you didn't use a path, then:
- In the car image: Select>Save to channel, this create a "Selection" channel
- Drag that channel to the canvas of the sea image, where it appears as a new layer
- Make it the only visible layer (shift-click its eye icon)
- In the Channels list, right-click any of the R, G, B channels, and Channel to selection
- You can now remove the copied layer
- You can use the Move/Scale tools in "Selection" mode to adjust the selection mask