Here's a question: if you change the scenario slightly, from being assaulted by a thug to being drowned by a river, does this change your response and the logic behind it? Why or why not?
I can't see that anything is different except each respondant's assessment of personal risk will vary depending on their relative skill level in each of those two situations.
In any case I would be doing a rapid risk analysis and going with my gut - there are times I have interceded/will intercede and times I didn't/won't - personally I'd be less likely to jump in the river because I think my risk of throwing my life away in vain is much greater in the river. Based on my life experience to date I have much more faith in my ability to handle human situations than to swim that well in a river (don't get me started on the people who drown trying to save their dogs...).