Right now, the logic for the mount list creation only checks for Vashj'ir with the Subdued Seahorse. The default swim speed for the Abyssal Seahorse is set to 450%, so it always selects it when swimming.

Thanks. I never actually got one, so I assumed it would be usable everywhere (otherwise it seems fairly pointless).

Edit: After looking at the logic, the Abyssal Seahorse should automatically be ignored in other zones by virtue of being unusable. Does /dump (select(5, GetMountInfoByID(232))) now return "true" outside of Vashj'ir?

That's the mount's type, not its ID.

However, these both return true and casting it outputs "You are in the wrong zone." to the UIErrorsFrame:
/dump (select(5, C_MountJournal.GetMountInfoByID(373)))
/dump (IsUsableSpell(75207))