he wouldn't have an advantage in the water so that's out of the question
he should make use of whatever he has in his current position which are himself, his clothes, the snake, the tree branch

As its unclear if the snake is a coiling snake (pyton) or one that is venemous, the steps with the highest chance of survival are:

1. climb to an opposite branch
2. use cloth from pants to restrict blood supply to his non-dominant hand
3. use non-dominant hand to distract snake while trying to break the branch
4. if successful in either breaking the branch or being able to lift the snake off from the branch, throw object at the lion
5. immediately jump down and grab rilfe
6. if rifle is loaded, shoot lion. then snake, then run from alligator
7. if rifle is not loaded, well, he's screwed.