The waters in Rio are unsafe for human consumption, and swimmers who enter the water are putting their health at risk. For this reason, athletes for water sports should not participate in the Rio Olympics. It was irresponsible for Rio to even put in a bid for the Olympics without the resources to make it happen safely.
Yes, unsafe water in Rio means swimmers should refuse to compete in the Olympics. As athletes, swimmers have spent their lives training to be in peak physical shape and good health. Unsafe water would threaten the high standard of health they have worked so hard to maintain. It is the responsibility of the hosting nation to provide safe, fair conditions for competing athletes. Brazil has failed to do so, endangering not only athletes' health but their right to a safe and fair competition. Athletes should boycott the Olympics until standards are improved.
The news stories about unsafe water are most often about untreated water in lakes and rivers. The water the Olympic athletes swim in is properly treated and tested before any competitions begin. There have been no news stories or reasons to suspect the water athletes compete in is contaminated in any way.
Rio promised to have the polluted water surrounding the city cleaned up by the start of the Olympics. It has been well known by the Associated Press, and their frequent testing of the water, that the water continues to be highly polluted. The swimmers have every right to choose not to swim in the polluted waters, however they have had sufficient warning to pull out. If they choose to not compete right before the Olympics, that indicates poor sportsmanship and poor judgement. The choice to pull out should have been made long ago.