No idea what it is, but the head reminds me of a dugong.
Apparently the boys who found it, kill it with rocks after they claim it came towards them, then threw it in the water.
Why on earth would your first reaction to something unique be to kill it? Idiots!
Could be a terrestrial animal post death, hard to say from one dodgy pic where you cant judge size or see the whole body.
I think it´s a Phyllophaga/Folivora can´t remember their common name... (a two toed sloth?)
but I think it have ben washed out in to the sea from a river