How big can an underwater animal really get before physics get in the way?  (feeding would likely get in the way first).  

The [Blue Whale][1] is currently the largest animal on planet earth the record holder 108 ft. long.  The [Megalodon][2] topped out close to 70 ft. [The Lion's mane Jellyfish][3] has been measured to 120 ft. long though masses much smaller than either of the other 2.

So how big in ocean's as large as ours, could an animal conceivably be?  And does the size of the planet make a difference?  Would life inside Titan be able to be larger? 


  [1]: https://en.wikipedia.org/wiki/Blue_whale
  [2]: https://en.wikipedia.org/wiki/Largest_prehistoric_animals#Mackerel_sharks_.28Lamniformes.29
  [3]: https://en.wikipedia.org/wiki/Lion%27s_mane_jellyfish