I decided to colour this instead of just throwing colours on them.

General:
Diet: seaponies are mostly carnivores, eating fish, seals, seafood and really anything meat (though not ponies). they also eat seaweed and a rice like fungus. they can’t eat hay.
livespan: do to the hard live underwater and the current Queens believe in survive of the fittest. seaponies live only half the time land ponies do. due to the rushed nature in with the seaponies were form their rates of birth defects are higher as well.

Earth Based Seaponies (black):
Size: Largest of the races, at about 18hands at the shoulder and around 135 hands (45’) long. they way it at around 4 tons.
Magic: like Earth ponies their magic helps heal and strength. they are also the only seaponies that can grow or farm anything. also like earth ponies they have no center in there magic.
Weakness: do to their huge size they can’t enter some of the under water cities.
Strengths: They are the strongest of the races and with their large size can beat off most of the monsters in the sea.

Unicorn based Seaponies (gray):
Size: smallest of the seaponies, at around 6 hands and around 45 hands (15’) long, they rarely weigh over 2 ton.
Magic: like unicorns they can use magic though their magic seems weaker, this is most likely do to the lack of horn though somethings one is born with a very small one. they can also use their magic to breath out of water, something that the other races can’t. their magic is centered in their gills and body.
Weakness: due to the spell used to change seaponies into what they are today, the magic levels in seaponies can cause some parts of it to fail, the biggest killer being gill collapses where their magic seals them. Being the smallest the also get eaten more often. Unicorns are rarer do to this only about 1/100.
Strengths: since they can breath on land and move around pretty well, they are usually what most thing seaponies are. some living in some of the port cities, also they size and large fins make them good crafts ponies.

Pegusus based Seaponies (orange):
Size: roughly the same size of unicorn based,
Magic: centered in their “wings” and it allows them great contorl over water flows, allowing them to be the fastest thing underwater.
Weakness: losing any of their fins makes it dangerous to go at full speed, they usually died after losing one.
Strengths: are super fast, and can hold their breath for a lose time, so they can go onto land for a little bit.

Alicorn based Seaponies or queen (purple):
Size: around 25hands at the shoulder and 168 hands long, weight in at 5-6tons.
Magic: the can breath air like unicorn, control water like pegusus.
Weakness: their size makes it hard to enter some of the smaller cities, their is also never more them one, if the current one dies a new one appears in the population.
Strengths: being op underwater but one land they suck.

my seaponies do got fins it other places this are just bald lol.