How do jellyfish overcome osmosis?
It is because the sea water seems isotonic to them.
For many marine invertebrates, like the jellyfish... osmosis causes no problems for them, simply because the concentration of solutes inside their cells, matches that of the concentration in the sea water they reside. This means that there is no net movement of water in and out of the organisms cells.
This video discusses the changes which occur in plant cells when they are placed into hypertonic and hypotonic solutions. Not exactly jellyfish, but the principles of what is going on is the same.
Hope this helps!