Complex sentence: It is a different entity than the United States Navy reserve fleets--the NDRF consists primarily of merchant ships, while the Navy version consists of warships, however the NDRF does maintain under federal contract a flotilla of warships in reserve status for the Navy.
Split into simpler sentences: It is a different entity than the United States Navy reserve fleets--the NDRF consists primarily of merchant ships, while the Navy version consists of warships. However, the NDRF does maintain under federal contract a flotilla of warships in reserve status for the Navy.