The U.S. Merchant Marine had the highest casualty rate of any American service during World War II, with approximately 1 in 26 mariners killed. This translates to around 9,300 deaths out of 243,000 mariners. The Merchant Marine played a vital role in supplying troops and materials, but their ships were vulnerable to enemy attacks, resulting in a high number of casualties.
Thanks; this is the first time I am hearing of this. (If you add the missing/presumed dead to your figure, the number is even higher.)
But the casualty rates among the American bomber crews were upwards of 60%. (It's correct that this statistic gets hidden in the broader number of US Army Air Corps/Force personnel, but statistically speaking, it was more dangerous to be in a bomber over Europe than it was to be a merchant mariner on a ship.)