The way I started the process with this crater is by taking inspiration from a real crater, particularly the image in the bottom right.
This, by process of using the threshold effect on gimp plus some additional drawing, we get this image which forms the base of the crater.
This image, by use of a plugin in JOSM, can be placed in JOSM and traced. I think there might be an easier way to trace it in JOSM other than doing it by hand, but that's the way I did it.
This comes up with the resulting final sea-crater-thing.