A leatherback turtle (Dermochelys coriaceaeating) eating thimble jellyfish (Linuche unguiculata)
This is a great video accompanying a nice paper on turtles and jellies [here]. Leatherbacks are obligate predators of jellies. Despite their size, leatherbacks are also difficult to study. Using planes to map out large swarms of jellyfish, scientists were able to predict the location of up to 22% of turtle sightings [paper]. In other words, find lots of jellies, find leatherbacks!
Thanks Arash for asking a great question and inspiring this post!
The Helm Lab Blog 
Wow! Jellies are so amazing! I’ll bet the turtle had a nice snack. Do jellies travel together like that all the time in one big group? Or do some species travel by themselves?
good question. Jellies can be found together for several reasons. One, they may all bud off in the same place, such as a bay. Two, as in this case, they can be concentrated by water currents into a single spot. Three, some jellies may actively travel in pairs, at least during mating. But this last one has only been documented for a few species.