The US sub captain did make a personal apology to the captain of the ship and the relatives of those who died. It was a personal apology. The US also formally apologized to the country of Japan.
If there was any delay or reluctance on the part of the Captain of the sub in apologizing it was because he is also facing charges for the incident.
CNN
The commander of the USS Greeneville Wednesday told the captain of the Japanese ship his vessel sank that he was "truly sorrowful and regretful" for the accident.
BBC
I'd like to extend my sincere apology," Commander Scott Waddle told them through an interpreter during a private meeting. "I can't ask for forgiveness. This is a burden I will carry to the grave."
yahoo
PEARL HARBOR, Hawaii (AP) - Three admirals who conducted an inquiry into the sinking of a Japanese fishing vessel by the USS Greeneville (news - web sites) will recommend Friday what, if any, disciplinary action officers on the submarine should face, a Navy official said.
The US letter to China was crafter very carefully so China's leaders could construe it as an apology. There are many different meanings of the word sorry in Chinese. My interpretation is that we expressed regret that their inept pilot rammed our plane and killed himself. We also said we were sorry that China did not respond to our Mayday calls on an international emergency frequency and our plane was forced to land at their airport without their permission.
yahoo
WASHINGTON (Reuters) - A crippled U.S. intelligence-gathering plane repeatedly sought permission to make an emergency landing in China after colliding with an interceptor but did not hear a response, Defense Department accounts showed on Friday.
My opinion.
If the submarine had been Chinese and trawler American the Chinese would have demanded an apology from the US for damaging their sub..