Jacob Marley
Adventurer
And yet create water is very definitely on the spell list for water priests in the original 2e rulebook (and again in the revised 2e rulebook).
You are right on the specific example of Dark Sun (I had to look it up when I got home from work.). The broader point still remains, though: banning spells does not mean an incapable DM. It could mean a DM attempting to create a specific environment or a unique challenge for the players.