There is a precedent for it though isn't there?
USS Stark was attacked by the Iraqi's in 1987 and USS Liberty was attacked by the Israeli's in 1967.
Both a Democratic and Republican president allowed such an act to slide with only a token response.
Mind you, as you mentioned in the current political environment Obama could not afford to act as all the nutters would be frothing at the mouth for revenge regardless of the consequences.
+1 to you for being well informed.
As for the USS Stark: well clearly Ronald Reagan hated America.

Seriously though, it sounds like that may have been an accident and it's unclear whether it was an authorized attack. Obviously it was a tragedy given that people died, but if there was no intent to attack then I can understand the desire to not escalate the conflict. Starting a war probably wouldn't have been productive.
Similarlyth e USS Liberty inquiry concluded it was a case of mistaken identity. Not productive to respond with military action.
Essentially, I'm saying that if it was clear that North Korea attacked us and they meant to attack us that the President would respond with military force of some kind. Suggestions that he'd wring his hands and try to start a U.N. Resolution are not only absurdly unfounded but are emblematic as to why this country is presently unable to govern itself in a civilized manner.