I wish I could have chosen "we can't know yet," so I erred on the side of caution. Ships sink for a variety of reasons, while North Korea -- while aggressive -- is not in the habit of sinking them. It may very well be that N. Korea is behind the sinking, just playing the odds points to no. N. Korea is not responsible for every Korean car accident or house fire, so there's no reason to assume they're behind accidents at sea until the evidence suggests otherwise.
Until the sinking of a South Korean ship has been fully investigated I find it unruly to shoot judgements over to another party. Yes, we all know that North Korea does not get along with South Korea but that does not say that they sunk a ship. It is all mere speculation at this point.
Given that the ship is sinking at the very southern tip of South Korea it is very unlikely that North Korea is involved with this ships problems. I suppose one could point to China, but I think that is even more unlikely. Some disasters are simply accidents, or the result of bad timing. I think this is the case here.
I don't believe North Korea is behind it simply because North Koreans, so far, were not known to be close by when it happened. North Korea's tactics are generally boastful rhetoric and they rarely take war-like action anymore in modern times. This is most likely just an unfortunate mishap due to marine causes.