If it were unarmed I would choose the croc, but since you said 1 melee weapon of our choice, and since you put the croc battlefield in waist deep water, I'll definitely take the bear and a stout spear.
The way primitive peoples did it was to get the bear to rear up on its hind legs in that intimidation posture they do and then try to get into position with the spear so that the bear falls on it; using it's own weight to help kill it.
I definitely agree. People have survived croc attacks, but it's not a very high percentage.
Probably should of said non-motorized weapons as well, but to late! I think the chainsaw does give you a fighting chance against the momma bear, but only a chance. I think your best bet is to hope momma just wants to leave with her cubs safely.
The one things about the Croc is, the battle begins in waist deep water, but there's no reason you can't make it to shore or up a tree. So there is a viable way to escape thus winning. It's just not very likely once the croc get's you. The bear, on the other hand, can and likely win follow you where ever so killing her is probably the only way to survive.
Another thing to consider is that, while the fight is 1 vs. 1, Nile Crocs (Unlike Salt Water Crocs) live in, I believe it's called a gang, so there will be other crocs there cutting off your escape...
A female coastal grizz weighs 500 lbs and a nile croc weighs around 900. Both are super aggressive. I would have to say I would rather fight a grizzly based solely on the fact that the croc is in waist deep water. If you could somehow escape the water I would take the croc then. They are much slower and clumsier on land.
Haha the water element is what makes it suck. Crocs on land go about 7-9 mph in extremely short bursts. In water they are near the 23 mph mark. A grizz goes 35 mph and those bitches will chase you everywhere.