Thunderdrum
These Tidal Class Dragons are known to be some of the largest and oldest dragons recorded. With an estimated lifespan of nearly three centuries these dragons can grow to compete with the size of a Heavy Battlecruiser alone, and a pod can compete with the size of an entire fleet.
As hatchlings, Thunderdrums are small with their bodies measuring about 11 inches long and wide and equipped with four stubby legs, two wings, and a long flexible tail. At this point in their life they can survive on land as well as underwater.
At age 16 A Thunderdrum is about thrice the size of an adult human, not including wingspan and their tail. At this point the dragon has reached maturity and grown several spines on their backs and tails. The Dragons stays the same relative size for about ninety years. During which they can live among humans and out of water without much problem.
After said ninety years or after the Thunderdrum's human passes away, the dragon retires back out to sea and dives deep before hitting another growth spurt one that makes the already imposing dragon a true titan.
These "Elder" Thunderdrums lose their legs and live completely underwater, only coming up to the surface on occasion to breach like wales. Probably to remember what it was like to fly all those years ago.
The Thunderdrum's namesake, it's sonic roar is achieved by inhaling large volumes of air in it's lungs and unleashing it by shouting. At maturity (Ages 16 to 106) the roar is powerful enough to shatter eardrums and send opponents flying with enough force to break bones. Elder Thunderdrums however are much more powerful, able to shred entire fleets of Longships with ease making them an incredible asset to the Corps Navy.
A Thunderdrum's diet consists of the expected marine food sources. but it is unknown what exactly they feed on in the dark depths of the ocean when growing into elders.
As for personality, Thunderdrums are more predictable than other dragons, Hatchlings are troublemakers without supervision. Mature Thunderdrums are stubborn at times, but unflinchingly loyal to their humans and each other. And Elder Thunderdrums are much slower in terms of response but are still unflinchingly loyal to their own kind and won't hesitate to help human in trouble at sea.
Thunderdrums are very trusting in each other and in their human allies, as a Hatchling will obey any mature Thunderdrum whether it's their parent or not. And Mature Thunderdrums have a mutual trust so strong with each other that one thunderdrum can walk into another's home (so to speak) and there would be no confrontation or standoff between the two.
Interestingly, the Egg of the Thunderdrum had a much stronger shell then most dragon eggs possibly to resist the pressure of deep water, and as such a Hatchling actually has to blast it's own egg open to hatch.
Notes:
An Elder Thunderdrum is able to identify the descendants of their former human partner and approach the descendents to aid them in said partner's memory.
Thunderdrums are protective of hatchlings and children even if it's not their own species, unleashing their wrath on any who attempt harm on any young life.
Hearing a Pod of Thunderdrums sing from underwater is said among sailors to be a sign of good fortune, as it implies that the pod is there to protect sailors from harm.