Although there are some great answers - I would argue that this question as worded today is:
- unanswerable in the sense that the question is too vague and each answer provided so far is clearly very different and none of them can be chosen as the answer to that question.
- insufficiently specific, not abundantly useful to others, and lacks "homework" or research to frame the question and make it clear to all what level of experience and what context this question was asked.
- far too close to bad-subjective when it could easily shaped into a good subjective question.
I would like to expand a bit and play devil's advocate.
Wouldn't this question be so much better and more instructive if the OP detailed what he (or she) meant by survival conditions?
Why exactly must one cross the river? Specifying that will go great lengths to constraining the answer and illuminating why the chosen strategy is simply the best practical answer to this obviously theoretical question. What is this other than a discussion at this point?
I would say we really have a poll question asking for and and all river crossing hypothetical situations.
berry120 does some convincing the OP not to cross the river, then lays out the necessary detail that should be included in the question and then gives some great advice. It makes many assumptions (rope, heating supplies, ) so it can't spend answer time on the critical parts of the answer - how to choose the time and place of a crossing.
Bror Johnson also has to first establish a context - and it's subjectively different than the other answers. It gives advice but doesn't explain why other than asserting experience in Swedish mountains. We can't know if the OP is in Puerto Rico where a very cold river flows down El Junque but no-one would worry about stripping down since the cold water isn't an element of the survival situation. Due to lack of detail, answers don't know the context of this question and can't give the best answer both for the OP and for others to learn.
Lagerbaer similarly comes up with different constraints with a group of three or more people crossing the river together. Is this answer wrong? Are the others right? Again, since the question isn't sharp enough, Lagerbaer returns to arguing against the premise that due to survival, a crossing must take place.
I don't think this is the worst question ever or the site is in dire need of closing it. I do think it could very easily be edited into something far more polished and useful and really something for the site to be proud of once as we seek to attract users. The sooner the site starts self-editing loose questions or closing them until they get shaped up, the better I see things rolling out. I could be wrong, though so do let me know how you all feel about this.