Sounds like you still need to tget the key from Kreug. He's the one that askes you if your there to join the serpent temple and requires 1000gp. With that key you can open both of the water passage doors and get the First Idol. The Center gate where you saw the Naga walk by cannot be opened from that side. You use that later after you kill the Naga adn Priestess and obtain the Mask of Evil.
