I always thought it was safe to assume three things:
1. Half-breeds existed only by crossing something with a human. Therefore, there were no half-breeds before Estarra's return. As per quote:
Despite interbreeding with humans, merians blah blah...

2. Most of Old Celestians were merian, so spectres from the island are also merian.
3. Purebred Merian had no hair, only fins. Because they are humanoid fish, duh! Hair = halfbred. Merians might not recognize them but I do. Descriptions in the Taint Wars history section and the official race picture seem to confirm this.

So, here we have a spectre:

Unaware of who or what he once was, this spectral man is clothed in ancient
rags. His eyes have become cloudy like eggshells, no pupils apparent on the
glowing white globes. He floats around with an appearance of despair and
melancholy, as if he has lost someone close to him. His hair sways in the air as
if it was floating in water, standing on end.

!! The hell. So, do Merians (purebred! no mongrels) have hair or not?

It's a metaphor. The shredded fins around his head appear to be hair, though in fact it is not true hair.

I dont think your ever going to find perfect continuity within the story and world setting. One misfit here and there doesn't mean much.

edit: oh shiz, estarra ninja'd me

Well, that was a quick answer.

Estarra loves us enough to clear up little things

Is there any proof that it's not, say...a dwarf? Most of Celest might have been merian, but all of it?

Well, since all the spectres of one type (men, women, etc) look the same, and most old Celestians were merians, then the description should, and does, give us an impression of a typical celestian citizen. That is merian, and not dwarf, dracnari, smurf or whomever.

So maybe they're a whole slew of different races? Cause, though most of Old Celest was merian, that makes it sound like a majority of them dove into the sea and swam away leaving only a few merian, and all non-merian.

The Olde Celestians left in a panic, tsk forgetting about those who cant swim! So cruel!

Usually, stuff like "classic" undead never really appear to have racial traits--they just become their monster type, a "generic" humanoid.

Plus, a spectre is considered the disemboddied spirit of a being--who's to say a ghost looks anything like the body?

Turn those vast observative powers into more efficient channels, and I think you can solve many of our problems in Lusternia.

If you live on an island, its your responcibility to learn how to swim

Coulda helped those that couldnt escape

Could've let the drown just as easy.

Wouldnt have survived either way.

Well, between being Aquamancer or Celestine or Merian or having a waterwalk/waterbreath enchantment, perhaps it was better that the people who didn't manage to have any of those aquatic abilities while surrounded by water were left behind. Lets call it evolution.

Not everyone's a member of a guild, Acrune. I imagine the majority of each city or commune's virtual 'population' consists of regular commoners.

the people of Old Celest were asshats. not that much has changed

I could so see Verithrax's great great to the n'th grandfather selling waterbreathe enchantments at the docks of Old Celest for 10,000 per charge during the evacuation.

Isn't Verithrax a human?