Estate agent Maria Pantazopoulos, 30, was apparently pulled under the water when her dress became saturated.
She is thought to have been staging a “trash the dress” photoshoot, where a newlywed bride is pictured ruining her wedding outfit, local media say.
Photographer Louis Pagakis said he jumped in to try to save her, but could not stop her from drowning.
“She had her wedding dress on and she said ‘take some pictures of me while I swim a little bit in the lake,’” Mr Pagakis told CTV News.
“She went in and her dress got heavy, I tried everything I could to save her. I jumped in, I was screaming and yelling. We tried our best.”
Ms Pantazopoulos’s body was recovered several hours later from the water, near the Dorwin Falls in Rawdon.