Water. With all genitals you are only meant to use water, you're messing up the pH and chemical balance of these areas and making it worse. Hence getting stuff like hemorrhoids or yeast infections.

I think the better thing to focus on here is that you should not have any poop to wash off in the shower, because ideally you don't have any poop stuck to you at all and you clean immediately on pooping.

If you are walking around like this, please -

1. Squat poop - more natural position, butt opens up, poop goes straight out and doesn't get smeared.
2. Use bidet, or wet toilet paper and wipe. Again, for anything that might be on your skin, don't just smear stuff around to sit there till your next shower.
3. If needed - cut ass air. There should be no "poop tangle knots" like someone mentioned, that is not ideal.