Five people from the UK died and an Australian man is still missing after the vessel, Leviathan II, sank near Tofino, British Columbia on Sunday.
The nurse, Sheila Simpson, who is originally from outside Strabane, County Tyrone, helped to comfort survivors at a nearby dock.
She said they appeared "shell-shocked".
Speaking to the BBC's Good Morning Ulster programme, Ms Simpson said: "God put me there on the dock, and thank God for my training in Omagh [County Tyrone] and in the Royal [Victoria Hospital] in Belfast.
"I just stepped into the place where I could be of assistance.
"The survivors who could walk up off the deck were shell-shocked and I looked them in the eye and I put my hand to their back and I said 'you are alive, you have survived'."
The nurse said she tried to provide "reassurance" to the traumatised passengers, adding that her experience of nursing in Northern Ireland meant she understood how "trauma stays" with survivors.
The victims have been named as David Thomas and his son Stephen from Swindon; Nigel Hooker from Southampton; Salford-born Jack Slater who had lived in Canada for many years, and Katie Taylor who was living in Whistler, British Columbia.
The missing man is 27 and a recovery operation is continuing.
Canadian investigators have said most of the passengers on the tourist boat were standing on the left side of the vessel when a wave hit it from the right.
They said the boat tilted before capsizing.
The passengers who survived were all in an enclosed part of the vessel.
There were 27 people on board in total and Canadian officials have said more could have died had it not been for the "amazing response" from locals around Tofino.
Fishing boats and other private vessels in the area helped to rescue survivors
A Northern Ireland-born nurse has described how she helped survivors of a drowning tragedy after a whale-watching boat sank off the coast of Canada.