Great book! :D

So the most important thing going on here is actually source bias. Hanawalt's major base of sources for this section of the book are coroners' reports from late medieval England. "Coroner" in this sense is a royal official whose job is to investigate unexpected death ("lay dead of a death other than his natural death", that is, not sickness or old age) and try to determine the facts of the case. Thus, her pool of cases is people who died in accidents or were murdered.

In that particular light, we can compare drowning deaths to overall 'suspicious' deaths at other times and places. Studying the Bills of Mortality from 1654-1735 England, a bit later, Craig Spence observed that 42.4% of accidental deaths were due to drowning! The next most common cause, falls, caused 11% of accidental deaths.

Spence points out that of the causes of accidental death, drowning was perhaps the most "ecumenical" in that it wasn't as closely tied to an activity that would mostly (but not exclusively) affect one sector of the population. People employed in various construction-related trades would be a lot more likely to die of falls (off platforms, ladders, etc). Workers in brewing and distilling were felled by scalding...and so forth. Factors like these would seem to carry over to the medieval cases as well--a lot of the drowning ones are, like, a young child bends over a creek to fill a cup of water to drink but falls in.

Incidentally, you can read several volumes of coroners' reports online--some in original French and Latin, but some are available in English translation! Here's one from archive.org. Enjoy!