The US Military is already heavily involved in disaster relief in Puerto Rico:

Navy, Marine Corps Providing Around-the-Clock Hurricane Maria Relief

Military conducting multiple operations to help Hurricane Maria-struck Puerto Rico, Virgin Islands

The Marines, Navy, Army, Coast Guard, and Air Force are all involved. This includes active duty, reserve, and National Guard units.

EDIT: Note that the Marine Corps has a the capability to provide potable water, using either sea water or polluted sources. These systems are already being put to use: Joint Task Force - Leeward Islands distributes water to the people of Saint Martin