Even if the ship-chaining up plot didn't work, Wu/Shu would have come up with another scheme to take him out. Though Cao Cao had more men, their morale is low. Most of his naval forces were newly-acquired from Liu Zong, and not well integrated with the rest of his troops. Jingzhou behind him was not stable. If Wu/Shu decided to dig in and do a drawn-out battle, Cao Cao would have been screwed--it's not easy to feed 830k troops (or even the 200k that he had in history)! There was a famine back home before he left, and he even had to ban alcohol before he left in order to save up grains for the campaign. The northerners in his army were bound to get sick at some point because of the climate differences and stuff (well, they *did* get sick too). Wu and Shu had a morale advantage because they're fighting for survival.
Basically I'm just reiterating Zhou Yu's analysis, but really, Cao Cao's pressing onto the Southland right after taking Jingzhou was a Bad Idea.
"Whatever you do, don't fall off the bridge! It'll be a pain to try to get back up again." - Private, DW 8