I wouldn't try to bake a potato but maybe you could nuke a hot dog. I don't know who might even try it unless it were some scenario like in the Tom Hanks movie Castaway where he survived on a tropical island following the crash of a FedEx plane by using the items that washed up. Maybe if a microwave washed ashore and it was in a place where you could catch electric eels.
A small microwave uses around 500-600 watts. IF an electric eel generated A/C instead of D/C, it would be able to generate 500 watts for about 1/4 of a second. No, you couldn't cook anything with it.