Spilled water is also likely to lead to an outbreak of coccidiosis. I'm not sure whether the OP was specifically losing chicks to drowning, chilling from getting wet or more likely coccidiosis from having a warm damp environment where cocci were thriving. Preventing water spillage is certainly a very important aspect though. I have broody reared chicks so water spillage is less important and much less risk of coccidiosis but I put a mixture of large and small rocks in a plastic tub for chicks to prevent drowning.