Galrahn, David Axe, and the folks at the USNI blog are closely covering US maritime assistance to Haiti. Both the USN and the USCG are on the way; the Coast Guard is responding first, largely due to proximity and preparedness. In terms of specific ships, USS Carl Vinson, USS Bataan, USS Fort McHenry, and USS Carter Hall all appear to be on the way. The hospital ship USNS Comfort is also responding.
For a case study of how US amphibious/maritime forces can respond to disaster, I recommend Bruce Elleman’s Waves of Hope, which examines the US response to the 2004 tsunami.