This document should be of interest for the immediate pre-WW2 period, as well as background info.
Water Transportation - U.S. Army, 1939-1942, ASF
As an example,
In 1914 the Army Transport Service owned 18 ocean-going vessels consisting of an Atlantic and a Pacific fleet together with a Philippine inter-island service.
The Harbor Boat Service in the same year comprised 130 owned vessels.
In 1939 the Army Transport Service owned some 32 transports, while the Harbor Boat Service consisted of 300 boats as of July 1.