Ask, and ye shall receive.
The last sentence of the article linked below mentions the contents of the container. Ironically, it may or may not contain harmful contents, depending upon whether you believe NPR or BoingBoing.
"And containers carry everything, from toxic chemicals to ribbon. The Monterey Bay container appears to be safe, though — according to the shipping company, it's full of radial tires."
Boing Boing rebuts:
"But some objects do a better job of functioning as healthy artificial reefs than others. The aforementioned pile of tires, for instance, was a popular idea in the 1970s. Later, though, it turned out that sunken tires were leaching plastic compounds called PCBs into the water. Those compounds can cause cancer in animals, so the tire reefs, while attracting wildlife, were also, probably, killing some of that life over time."