They kill plenty of alligators with .22s, but those shots are carefully selected and you wouldn't have time for that. I would personally prefer a .45 for gators.
Bobcats are extremely unlikely to bother you. They are too small to consider adult humans prey. I saw a nice one in Punta Gorda a few months ago and he took off full speed the second he realized I was looking at him.
99% of the time, if you see a snake and have time to shoot it, you have time to avoid it. The exception I can envision would be if you stepped too close before realizing and it would be unsafe to move due to risk of provoking a strike. In that case, any caliber would be fine.
Note that most wildlife is protected, either by hunting regulations or conservation laws. If you shoot something out of season or protected, be prepared to face charges unless you can prove it was necessary.
Coyotes in some areas are becoming a threat to humans, especially kids, but most places they are afraid of humans. Bears can be a remotely possible issue, but a .45 would again be my choice of what you have available.
I would be much more concerned with human predators, in which case the .45 would still work, as would a 9mm or 5.7.