In fables, dragons are generally characterised as very intelligent creatures, thus they can adapt to environmental change better than other species.
In some stories they live in caves, in others on mountains with a nest like (giant) eagles.
Yours is seemingly the latter.
Why should an animal nest at such an remote location?
Dragons are an extremely dangerous animal and thus human will try to kill the brood before they become a threat. Nesting at the highest heights provides excellent safety, because the dangerous human has to take a high risk while climbing the mountain. Climbing mountains is dangerous in any case, but with a mother dragon that notices the climber, it's suicide. Climbing takes considerable time in which the mother dragon can notice the human(s) and repel the danger.
Why to hunt in deep sea?
Your dragon is larger than a bus. This means it needs a lot of food. A deer is more of a little snack than a meal and hunting in a forest is near to impossible for such a large creature. In open fields ( stealing humans' cattle ) it's dangerous, because the humans might figure out how to kill a dragon. Humans are highly intelligent in organizing as a group, using tools and weapons. AVOID!
Also, lizards and snakes ( Dragons are in the same biological family I would assume. ) need to rest after a larger meal; they cannot travel far for, at least, some hours. So it would be best if the prey could be brought to the nest to be eaten there.
But gathering 10 larger animals on land, like cows, bears or deer, would take the whole day. Better to find something bigger. If there are no rhinos or elephants, search has to be under water.
A group of white shark or tiger shark is a dangerous threat, even a large dragon could receive capital wounds which is critical for every predator.
A predator that cannot hunt (due to the wounds) will starve. The Mantis-Claw technique of your dragon focuses on individual prey, not groups.
Sperm wale and giant squids are both more dangerous than a white shark, but single individuals, not groups. For both, the right technique should be chosen. (However, there should be large prey near the surface, like orca, blue whale, whale shark and so on).
The pros of "hammerhead".
Same as the hammerhead shark, the dragon has a larger range of sensory perception and has a better 3D perception over long range. (Triangulation)
Also, it's easier to look beside corners and over cliffs without being seen, if you only have to show a little part of your hammerhead, not the entire head.
Some fish deal with this problem with their air-bladder.
Your dragon could have a hollow organ in which it could produce different pressures by producing specific gas. This hollow organ could be the lung, but then, the lung has to be very tough to not end in a pneumothorax then.