(Assuming the shower is running. Any temperature.)
The best I could come up with was a hot soup, but eventually you'll get more water then broth. There has to be a better food out there for those hungry showers!
Something dry that even in hot humid air won't spoil quickly.
Cheese seems like a good bet. Certain cooked and cured meats similarly; salami or beef jerky, cooked sausages etc.
But ultimately why would you eat in the shower? The real problem is either time management issue or eating too much of you really need to cram food in your gob while showering. Also would waste more water and time in the shower if you're both eating and cleaning yourself.
Its the sort of seemingly innocuous question that actually shows you maybe should be taking a long hard look at your life priorities. The shower is a good place to think about that, maybe while chewing on a block of cheddar cheese.