If given the chance to learn, they will choose to hold their poo for a while to get to the water. I used to tub-poop all my iguanas every day at 6:30 to 7:00 AM. One even started grunting and straining as soon as he saw the tub. Others started straining as soon as they saw I was moving them over the water. And others needed 20 minutes. Some of my rescue iguanas would even poop at the same time, side by side in the water.
I didn't let them explore in the bathroom. That kept it territory-neutral. I never had a nip or a fight between tub-mates.
We had a member who had a very relaxed iguana, and she had the time to train it, so we got pictures of her ig standing on the toilet seat, looking backward to check that it's vent was over water. I'm sure many iguanas could learn that. With careful and compassionate training.