First, Objectivist principles are intended to be rational guidelines for maximizing long-term happiness. Accidents and emergencies are, by definition, exceptional events, and behavioral principles aren't always applicable.
Secondly, Objectivism is against altruism in the sense of an obligation to sacrifice to others. It isn't globally opposed to acts of kindness or charity.
The traditional objectivist answer to the question is that an Objectivist bystander should make as well-informed assessment of the risk of providing aid, compared to the value the other's life holds in relation to the objectivist, and act accordingly. This judgement can't be made by anybody other than the individual, because only they know their values.
An objectivist who is drowning would call for help, in the hope that others would save him (i.e., believe that the drowner's value is worth the risk). It would be wrong for the drowner to believe that the bystanders had a moral duty to risk death to save him, but not wrong at all for him to appeal for freely given assistance.
Personally, I think that a practical objectivist incorporates a little bit of game theory into their behavior, and knows that a some minor assistance to others as a general rule can grease the spokes of society... again, as long as it's freely given and reciprocal.