According to Official Guinness Records,
The greatest distance by canoe/kayak in 24 hours on flowing water was 449.04 km (279.02 mi) by Carter Johnson (USA) down the Yukon River starting in Whitehorse, Yukon Territory, Canada, from 25-26 June 2011. Carter used two different GPS devices to record his attempt, a Garmin Oregon 450 and a Garmin Etrex. Carter was followed by a support team for the entire 24 hours. The members of the support team were Greg McHale and Mike Richards.
For a complete list of 2011 records, please visit 2011 Guinness Records in Canada.