How do manatees defend themselves against sharks?

ManaTeeKwanDo ?

Manatee's are fresh water, Sharks are salt water. They never encounter one another. Peoples boat propellers are the Manatee's biggest worry not Sharks

Well, I live next to the Gulf of Mexico - a salt water body - and there have been manatees spotted here, in my backyard!

from Wikipedia:

Although manatees have few natural predators (sharks, crocodiles, orcas, and alligators)

so this IS a valid question! Answer... hmmm, back to Wiki ;)

"However, they have been known to swim at up to 30 kilometres per hour (19 mph) in short bursts." this may have something to do with it, but still looking...

Ah. Here is the sad answer from:
http://www.manatee-world.com/manatee-predators.html
Many young manatees lose their lives in the first few years to these types of predators in the water. The mothers are extremely defensive of their young, but they don’t have much to fight with. They can sometimes get in a few hits with their tails. Since they can’t turn their heads without turning their bodies, they don’t have the speed either that they need to defend themselves. The teeth of a manatee are very dull so they don’t help with defense from predators either.

First of all Manatees are not only in fresh water. They live here in salt water also.
They don't have any defense at all and most every one you see is scarred horribly, some have chunks of skin missing, pieces of tail or flipper missing completely. At least a shark would be doing it to eat. Most of the injuries are due to man and boat propellers.

I think they hide.