Every account that looks at the life of the troops in the trenches mentions the danger of drowning in flooded trenches. There was a major flash flood disaster in the last stages of the Gallipoli campaign during which some sources state there were hundreds who drowned or froze to death. (See description below.) However, I never seen any authoritative statistics or analyses on the subject. I'd appreciate any readers who have seen such material posting the source on our comments section. Looking at the photos below, its quite credible to me that it's a very large number. The image from Suvla Bay, Gallipoli, is from after the flooding described.
On the Western Front
At Suvla Bay, Gallipoli
Shortly before the evacuation [from Gallipoli] a storm had swept over the Peninsula. First it had rained for two days, the third day it snowed, and the next it froze. A torrent of water had poured down the mountain side, flooding the trenches, and carrying with it blankest, equipment, rifles, portion of the parapet, and the dead bodies of men who had been drowned while they were sleeping.
Text from Trenching at Gallipoli by John Gallishaw, image from the Canadian, Newfoundland & Labrador Heritage Site.