The Associated Press reported that 13 members of the 40-member US team fell ill after the world junior championships, a test event for next year's Olympics in Brazil.
US officials confirmed to Reuters that 15 members took ill but said that was not unusual in international events and it was too early to blame dirty water.
"It would be easy but irresponsible for us to immediately assume that the rowing course is the main or sole point of exposure that caused the illnesses," Glenn Merry, CEO of USRowing, told Reuters in an email.
Merry added that US rowers often took ill abroad and said the fact that coaches also got sick in Rio was an indication water might not be the problem.
The only athlete who fell into the lagoon and consumed significant amounts of water was not one of those who fell ill, Merry added.
Canadian officials told Reuters that not one of their 24 athletes and support staff were sick.
"We are pleased to report that during the time the team has been here, we have had zero incidences of illness," Peter Cookson, high performance director for Rowing Canada Aviron, told Reuters.
"We took a number of additional measures [in addition to pre-travel vaccines/medications] such as protecting water bottles from exposure to the water of the lagoa [lagoon], covered all open scratches or sores, and washed with antiseptic soap after every session.
"While there were a few reported illnesses from some who stayed in the same hotel as us, we feel the extra precautions we took assisted in keeping our illness rate to zero."
Event organisers said they treated 14 people for diarrhea - eight Americans, and three each from Australia and Britain - and that all were medicated and fit enough to compete.
A spokesperson for Rio2016, who asked not to be named, said "everything suggests" the diarrhea was caused by familiar travel woes rather than dirty water.
The championships were held at the same lagoon that will be used during South America's first Olympic Games next year.
Water quality there, as well as in the seas where the sailing, triathlon and open water swimming events will be held, has been sharply criticised and authorities have admitted their own targets for reducing the amount of sewage in the water would not be met.
Unsafe levels of viruses and bacteria were recorded in the water, according to an independent study commissioned by the AP and released last month.
