By that logic, couldn't a waterbender just freeze all the fluids in ones body (and perhaps make all frozen parts shatter afterwards) to make someone die a very painful death?

For example: The brain is surrounded by the Cerebrospinal Fluid. If you could freeze that your opponent would die for sure.

I like the way you think.

Well, bloodbending is similar.

Not really, that's more like controlling the other person like a puppet.
I mean like really freezing them from the inside out.

They can only bend stuff that is considered water, unless they're a bloodbender, then blood too.

There is a lot of water in your body bro.