Sea Serpents are, potentially, the most powerful marine creatures in the whole world of Equis. Direct descendants of the ancients dragons, they inherited a similar strenght, power and intelligence, but adapted to the marine environment.
Sea serpents can be best described as gargantuan snakes, provided with arms and clawed hands like dragons, but with a generally more refined shape. Other physical features they can have in common with dragons, but not always, are the spikes on their backs, the fin like ears and the scale less stomach. The wedge at the end of the tail is sometimes replaced with a caudal finn. Unlike dragons, serpents usually do not have big spikes or horns on their head, but a thick hair and with age they can grow facial hairs like beard and mustaches.
Like dragons, their dimension change greatly with age; chubby and never longer than a couple of heads as hatchlings, when they reach almost a millenia of age (achieved by few) can be to a dragon what a whale is to a elephant, only longer. This similitude is more accurate than one could think: such sizes can be reached only by the ones living their whole lives on the deeps, where the pull of gravity is lessened by water (serpents living in rivers stop growing a lot before), like between mammals whales are bigger than elephants, ground-bonded, only because they live underwater. Always like whales, those greatest serpents cannot get close to the ground without being crushed by their own weight. So no snakes of doom for equestria. Such a shame.
Scales and fangs:
Their dragonic inheritance gave them dragon like scales, claws and fangs, with the difference they are usually not as hard and sharp. This weakness is because of the different diet: Sea serpents have not access to the same amount of gems, metals and refined minerals as dragons. This means they are actually able to crush metal and gems, but with more difficulty than dragon and, in parity of weight, even than diamond dogs.
Sea serpents have an extremely strong bond with water, to the point they consider it to be a sentient force to be loved and befriended. This bond makes them sensibly stronger and faster while being wet (no pun), and some amount of humidity is necessary for their survival. For this, even while out of water, their inner magic gather the water in the air around them, even of this is not enough to keep them going and every some time they need to enter in contact with liquid water to reestablish the right conditions. Serpents can use the same magic to generate light currents of water. Most of serpents, unlike dragons, usually not have any kind of breathing magic, but that can be developed after reaching a really old age (like the metal breathing of Rustjaw from Wild Card). Almost all, instead, have magical-augmented organs similar to the ones torpedos and electric eels use to generate a difference of potential and so a electric current. Such power is effective only underwater or in condition of great humidity, so the water can works as conduit for the electricity.
From dragons they inherited also the scales color hierarchy and bond characteristics, even if to a way lesser degree. For example, a red serpent is not so easy to anger like a red dragon, and a yellow serpent can be greedy but because serpents do not hoard it will not be obsessed so much by his possessions.
The most diffused breed of serpent is the blue one, thanks to the possibility to camouflage their color in the deep waters. On the other side, red and yellow ones are rarer as easier preys when too little to defend themselves. Finally, black serpents are so rare they are legendary: given the lack of minerals in their diet, is already hard for one to be born, and even if it was to happen the heavier scales makes a lot harder to swim making them easy preys and making hard for them to catch their meals.