Visitors should note that a dip in Amsterdam's canals is prohibited in most cases (save for one exception, described below). So unless a tourist wants to risk both a monetary fine and the potential safety threats it poses, it's wise to resist except in a few sanctioned scenarios.
Water Quality in Amsterdam Canals
Now on to safety. A report issued in 2007 states this:
"Testing of the canal water quality for compliance with the stringent standard values for fecal indicators in the revised European Bathing Water Directive, which came into force in 2006, has demonstrated that water quality did not comply with the standards. The canal water was therefore considered unsuitable for swimming and health risks for persons exposed to these waters cannot be ruled out."
In fact, until 2007, Amsterdam houseboats were not even connected to the city sewer system - which meant that their waste would be deposited directly into the famous canals. (The canal houses themselves were not fully connected until 1987.) Since then, however, Waternet - the city water authority - has closely monitored the water quality in Amsterdam's canals, and Radio Netherlands Worldwide reported as early as 2011 that the authority has seen a marked improvement thanks to their new sanitation measures.
Nevertheless, four years later, only a quarter of the city's characteristic houseboats had been connected to city sewers. It is hoped that all the city's houseboats will be connected in 2016.
There is also the concern of debris in the canals. Rubbish of all sorts finds its way into the city canals, from paper and plastic to bicycles and even the occasional car. Sharp points on these discarded items can pose a health hazard to swimmers.
Exceptions to the Rule: The Amsterday City Swim and Royal Amstel Swim
So then why did Queen Maxima - then still Princess Maxima - take to the waters in September 2012, clad in a wetsuit and swim cap? She and a thousand others were participants in the Amsterdam City Swim, an annual charity event in which a thousand fundraisers take a one-and-a-quarter-mile swim in the iconic canals. The Maxima's 2012 swim and the subsequent, 2013 edition of the Amsterdam City Swim raised money (and awareness) for ALS research. The route, which takes at least a half-hour to complete, proceeds from the IJ River - the body of water that separates Amsterdam North from the rest of the city - to the Amstel River, then backtracks up the Amstel to the finish line at Keizersgracht.
So while much of the swim technically takes place in the city's rivers, the final stretch takes swimmers into canal waters.
The Amsterdam City Swim takes special precautions to ensure the safety of its participants and the cleanliness of the water. Prior to the event, Waternet, the abovementioned city water authority, checks the waters extensively and removes debris from the course; if the water quality is still too low, canals are pumped with fresh water, or an alternative route is taken. Even so, swimmers are advised to wear a wetsuit, not to swallow any water and to have the appropriate vaccinations. If that doesn't put you off, you can find more details about the event at the Amsterdam City Swim website.
Somewhat lesser known is the Royal Amsterdam Swim, the oldest open-water swim event in the Netherlands, which also trumpets a worthy cause: awareness for clean water. The one-and-a-half-mile route travels from the Stopera, the city hall-cum-opera house on Waterlooplein (Waterloo Square), down the Amstel to the vicinity of the Amsterdam Amstel train station.