Thanks again to the marvel of Google, looks like you are 100% correct.
American merchant marine deaths were about one third higher than next highest, being USMC, according to US Merchant Marine website.
Tables are a bit out of alignment on pasting. Click on link for aligned tables.
U.S. Merchant Marine Casualties during World War II
Mariners suffered the highest rate of casualties of any service in World War II, but unfortunately, the U.S. Merchant Marine had no official historians and researchers, thus casualty statistics vary. Revised 08/26/06
Mariners died "direct result of enemy action or as POW on American flag ships" according to U.S. Coast Guard, 1950 (excluded Army Transport and U.S. owned foreign flag ships) 5,662
Mariners died compiled by Captain Arthur Moore ("Careless Word" 1998 edition) 6,847
Mariners died compiled by www.USMM.org 8,421^
^Additional 1,100 died of wounds later according to testimony before Congress.
Comparison of Merchant Marine casualty rate to other services
How many U.S. Merchant Marine and U.S. Army Transport Service casualties were there in World War II? We may never know the exact count, because the U.S. government never kept accurate records as it did for other services. The best previous accounting has been by Captain Arthur R. Moore, an independent researcher, in his great work, A Careless Word -- a Needless Sinking: A History of the Staggering Losses Suffered by the U.S. Merchant Marine, both in Ships and Personnel, during World War II, American Merchant Marine Museum, U.S. Merchant Marine Academy, Kings Point, N.Y., 1983 to 1998. Captain Moore's book lists approx. 990 ships. www.usmm.org list includes 1,600 ships.
Service Number serving War Dead Percent Ratio
Merchant Marine 243,000* 9,521** 3.90% 1 in 26
Marines 669,108 19,733 2.94% 1 in 34
Army 11,268,000 234,874 2.08% 1 in 48
Navy 4,183,466 36,958 0.88% 1 in 114
Coast Guard 242,093 574 0.24% 1 in 421
Total 16,576,667 295,790 1.78% 1 in 56
*Number varies by source and ranges from 215,000 to 285,000. War Shipping Administration Press Release 2514, January 1, 1946, lists 243,000
**Total killed at sea, POW killed, plus died from wounds ashore
http://www.usmm.org/casualty.html