The A on Aquaman's belt is a glyph which stands for Atlantis.
As shown here from Aquaman (New 52) #4:

It's explicitly mentioned in Aquaman (New 52) #5:

The only piece of actual Atlantean writing I've been able to find is by Atlanna (Aquaman's mother), taken from Aquaman: Time and Tide #1 (from 1993):

This appears to be using an English alphabet, suggesting that it is both the symbol for Atlantis, and an A.
This text came from the closing narrative of The Atlantis Chronicles #7 (from 1990). Here she is talking about the journal:

And writing in it:

I've not been able to see anything to suggest a distinctive Atlantean alphabet within DC Comics lore, such as there is for Kryptonian and Superman's "S".
IGN mentions this when discussing the film costume:
The "A" on his belt is of course the "A" for Aquaman. However, just as
Superman's "S" shield has gone on to become a family crest and a
symbol for hope, so has Aquaman's "A" in the comics. Firstly, the
symbol represents Atlantis in the Atlantean alphabet.