Of COURSE I have heard of bodies being put in containers. The point, from the murderer's perspective, is that the body should never been seen again. If the container weighs it down, hides it from view from above, and keeps it under the sea, that's one thing. But if you have an easier option with the sharks...
But, they would have to strip the body because sharks don't eat clothing...and also, they don't eat hair and some it might wash ashore.
Offshore, I suggest a container would be best. Something like a 55 gallon metal oil barrel with an anchor in the bottom. Seal the top tight and puncture it so water can seep in.
If not that, I think an abandoned well on the island would be the ideal place to hide a body.