There is indeed a significant disparity according to https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6319a2.htm https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6319a2.h

There is indeed a significant disparity according to https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6319a2.htm https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6319a2.htm although that study doesn't analyze what may cause it (other than differences in the prevalence of swimming training).