What sea animal is this?
It's probably a dugong, based on the location, lack of a dorsal fin, split tail, lack of a blowhole, and narrowing of the snout.

The prominent vertebral column looks unusual, but that might come from the strange posture it is being held in or from malnutrition.

This publication from the Australian government shows necropsy of several dugongs (note, this PDF contains graphic necropsy photographs), including one that was malnourished (see page 53). This individual shows some of the characteristics of the OP, including pronounced vertebral column and a clearly visible neck. The text notes the presence of these features as well.


  Adult dugongs showing signs of malnutrition: a) with neck becoming visible
  and prominent backbone 


Sadly, the other dugong pictures I found that look most like this specimen are also ones from news sources that are often dead or injured or needing some other human assistance. Some other examples:


Queensland couple come to rescue of beached dugong
Dugong dies from propeller wounds
Trang fishermen asked to help slow-moving male dugong
Rare Dugong saved in Maliangin Community Marine Sanctuary


Some pictures do show a prominent vertebral column near the tail in animals that seem otherwise healthy, but those images don't show the rest of the animal well (thanks to @theforestecologist):

http://www.oceanwideimages.com/images/11147/large/dugong-58M1444-20D.jpg
http://www.oceanwideimages.com/images/11144/large/dugong-58M1444-17D.jpg

As an extra piece of trivia, dugongs and their manatee cousins have been closely associated with mermaid legends.

Of course, a better picture of the animal's head, especially in profile, would make identification much more clear.