Elizabeth Morrissette told GMA people were panicking and yelling loudly as they were thrown into the water, which emergency officials said was between 1.5m to 1.8m deep.
Some passengers were able to get to safety. Emergency crews used an inflatable boat to rescue about 16 others, fire officials said.
City of Lockport Police were investigating the accident. An email seeking an update was sent to the police chief Tuesday.
The tours take visitors on an underground boat ride through a rough-hewn tunnel, which was blasted out in the 19th century to transport canal water as an industrial power source.
The attraction is about 32km northeast of Niagara Falls.