I imagine they'd see the poop coming and be able to catch it with a bag/gloves and chuck it straight in the loo. Or it'd float and they'd just scoop it out. I think someone would be there during your pushing stage, so I'm not sure the poop would get a chance to sink. You could always ask the midwife, I'm sure they'd have a plan.

If you don't want to waterbirth then don't push yourself for one - you want to birth in a way that's comfortable for you, not for others. (Although having said that I loved mine. ) It's great that you're keeping an open mind. xo