I think Al Jazeera was referencing this Vietnamese story:
Vietnam Navy says Malaysia Airlines plane crashes off Tho Chu Island | Tu?i Tr? news
The Admiral who was mentioned has since said that he had been misquoted and that his only reference had been the oil slick. So there is nothing factual. Even the oil slick may be a red herring, as it may not be from aviation fuel.
I'm astonished that a plane stuffed with all manner of floatable items can disappear so completely.