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What merits this award is not the “rate of loss” but the fact that these volunteers served without any defensive forces arrayed to protect them (with the exception of the later developed armed merchantmen. Here: https://en.wikipedia.org/wiki/Defensively_equipped_merchant_ship#United_States) It is facing high percentage of sinkings without any defenses that should be the honor bestowed on these volunteers! Not taking anything away from so many who did. But facts are important.

From this page: http://www.usmm.org/casualty.html

“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”
All the above being said:
The stated casualty RATE (rate is different than number or percent of total serving killed) vs. any BRANCH of the military. This is not true however vs. higher casualty numbers (as in, killed in combat or never found) in specific forces in branches of the military.

Two, to be exact: USAAF, and the US Navy Submarine Service (both theaters for each of these)

The US Army Air Forces.
Statistical Digest numbering of lost servicement is presented as “crew” losses instead of individual fatalities. 8,500 bomber crews were recorded as having been killed in the various WWII theaters of operations Adding the crew numbers from these losses is calculated at over 149,000 men.
In all theaters of WWII. The death rate difficult to obtain among all air crews, let alone talking about all serving members, ground and air. Example: In the The Eighth Air Force alone (over Europe)- out of a total 10,631 aircraft (B-17 or B-24, crewed with 10 or 11 each)... 4,145 were Lost: 41,450 to 45,595 killed,in mission losses. Thus, part of that 149,000 plus lost worldwide.

Here: https://archive.org/details/ArmyAirForcesStatisticalDigestWorldWarII/page/n5/mode/2up

Unites States Navy Submarine Service:

During World War II, the U.S. Navy’s submarine service suffered the highest casualty percentage of all the American Armed Forces, losing one in five submariners as killed/dead.
No American Armed Forces suffered as high.

Some 16,000 submariners served during the war, of whom 375 officers and 3131 enlisted men were killed,combat total 3506 (not including those killed in POW camps or other locations held) 336 submarines (not all of which were serving in Combat in either Atlantic or Pacific theaters). Lost during WWII: 60. 52 subs lost to combat.

Suffice to say— if one was a submariner in WWII, the highest possible chance of being killed. Period.

http://www.oneternalpatrol.com/sources.htm