I share your reluctance to use the fact that, as I generally find that its near redundancy can be made totally redundant with some gentle rephrasing. In this case, how about just avoiding the passive:
The article mentioned that the international waters are so vast that it would be impossible to use navies to protect the area.
Alternatively, maintaining the passive, you could use a nominalization instead of a subordinate clause:
Mentioned in the article was the impossibility of using navies to protect the area, as the international waters are so vast.
Or a more succinct version of the same structure, eschewing the as clause:
Mentioned in the article was the impossibility of using navies to protect so vast an expanse of international waters.