Can showering in very hot water make you skinnier?

You can try it, but if there's any effect it would be minimal. Cold water would be more effective because your body would burn fat to warm you up, but even this wouldn't make too much difference. The best way to lose weight would be gentle exercise 3-4 times a week with a diet low in fat but high in protein to help build muscle strength (the stronger your muscles are, the more fat they can burn for you)

Good luck.