Is Merchant Marine under Us Treasury Dept?

Answer 1

No,

in fact in spite of what you think, there is no organization called "the merchant marine"

The phrase merchant marine refers to an industry, just like the phrase commercial airlines refers to an industry.

The phrase merchant marine refers to the commercial shipping industry, ships like container ships, tankers, bulker, break bulk ships ect, tha are owned and operated by private companies like Matson Navigation, Pasha Lines, TOTE, ect.

Merchant mariners are civilian employees who work on merchant ships and just like there are legal requirements for those who work on aircraft there are legal requirements for merchant mariners, the federal agency that licenses merchant mariners is the U.S. Coast Guard.

The industry itself is regulated by the U.S. Coast Guard, The Maritime Administration and international agreements.

Now that you have that in mind, lets go into the area that leads to much confusion;

The federal government and many state and local governments also operate ships, and the people who work on these ships also have the same licenses as those who work on commercial ships., in many cases people move from one to the other, some military ships are even operated by private companies, in fat the military does not transport much of its own cargo, they depend on merchant mariners for that, both those employed directly by the U.S. government (called civmars) by government owned and ships operated by private companies, and by commercial ships.

This is why merchant mariners, though not part of the military had a higher Death Rate than the military during WW2, and for that reason in the 1980's merchant mariners who had sailed during WW2 were granted veteran's status.