Could Elodea or Paramecium from a freshwater lake be expected to survive if transplanted into the ocean. Why?

No. They are not evolved with a mechanism for dealing with the highly salinated water, and so they would shrink and die. This is because the salt in the water makes the sea water highly hypertonic to the elodea or paramecium cell, and it would result in a rapid loss of water from their cells as water flows from higher to lower areas of concentration.

This is why they tell people never to drink salt water. You'll die of thirst faster because you will make your blood hypertonic to your body cells and that will draw the water out of them and you'll die.

Salt water algae and other organisms have various mechanisms for pumping out the salts; they have evolved to exist in a marine environment. If you put them in fresh water, the opposite would happen. They would swell up and their cells would burst because the fresh water would be hypotonic to them. In short, you can't put fresh water organisms in salt water or vice versa or they will die. There are a few exceptions, but these organisms have evolved for this. An example is salmon, who live in the ocean as adults but return to the fresh water stream where they were spawned to reproduce, and the small salmon are born in fresh water. However, this is relatively rare and an exception to the principle.