Water alone cannot grow bacteria. They need a food source too, an energy source. Sunlight is probably not getting in, so where does the food come from? How do they fuel metabolism? Just like you, they need food and water.
“Dirty” water can grow bacteria, because there’s more than just a bunch of H2O molecules in it. Something like distilled cannot.
So, depending on the source of your water, you can be fine. Like, a lot of people use RO, and that’s pretty clean water.