since the Colonies are very much Sea only, how about Sirens or Sea/merponies?
I tested a few abilities on my server before thinking about suggesting it here.
Level 1 - > Seapony

(art credit to dizzyness@DA)
Seaponies are a kind independent Race within Equestria. Some say that Seaponies have been rescuing Ponies from drowning and brought them back to the beach before disappearing into the depths of the Ocean again. Seaponies use Magic evolving mostly about Water and Ice. It exists Rumors that a Seaponies Tear has the power to keep one young for an eternity.
Active Skills:
RQ LvL 1 /Max 2 - Freeze - Freezes Enemy for a short time and deals a small amount of Damage - Single Target
RQ LvL: 1 /Max 5 - Water Gun - Spit Water on your enemy and deals medium damage - Line Attack / Multiple targets
RQ LvL: 5 /Max 6 - Wrath of the Sea - Summon a Wave that pushes and deals a large amount of Damage in cost of your health. - Multible Targets / 30° Cone ( Requires to learn "Water Gun")
RQ LvL 2/Max 1 - Summon Fish - Summons 4 Raw Fish
RQ LvL 2/Max 5 Lifesteal - Melee attacks have a chance to steal health
Passive Skills:
RQ LvL 1/Max 1 - Water Creature - Allows to breathe and slowly regenerate Health while In water
RQ LvL 2/Max 3 - Swimm Speed - Increases the Swimm speed (I have it on my server but since they would be using "fly" in water and speed that I think it's pointless)
RQ LvL 1/Max 5 - Night Prowler - Do more damage at night - Night Time
Class Siren: (based of actual Myth and not EQG)
Req LvL 30
(Art Credit to negasun@DA)
The Siren is a mythical creature and often disguised as a beautiful Pony Being. Unlike Changelings they are not able to use Magic or the ability to fly when in Disguise. They say their voice can either Heal or Kill you. Unlike other Merpony kin, Sirens have deadly Fangs and strong Fins that can break a Strong Stallions bones with ease. Some ponies Say that Sirens are Corrupted Seaponies.
Active Skills:
Nightmare - Puts 5 Targets in a radius of 6 meters to sleep and adds a bleed effect - AoE
Lure - Pulls a target Close and Stuns them
Song of Healing - Heals 6 Allys including the caster in a range of 5 meters. - Buff AoE
Song of Unhealing - Deals a Large amount of Damage in a range of 6 meters - Attack AoE
Cry - Confuses the Target - Multiple Enemies / 30° Cone
Summon Water - summons 5 Water Blocks
Lifesteal or Leech - See existing Skills
Passive Skills:
Scales - Hardens your Scales and adds Defense
That's All i had in Mind right now. I wish i had more Passives for the Siren right now but Eh....
On my server I have Moth and Flutter ponies. They evolve from Earth Ponies. (I wish i was smarter enough to create their skills though..)
But I'd really like to see Seaponies/Sirens because of the idea of building an Underwater Cave Town for Them.
(I'd also really would like some feedback to that Idea... feel free to change things though, It's just ideas for Active and Passives)