OTL - HMCS Rainbow (Apollo class, 3,600 tons), Canada's first warship departed Britain in August 1910, arriving at CFB Esquimalt in November. She spend a few months in Britain beforehand for modifications, including the installation of the latest Marconi wireless systems.
ITTL, HMCS Rainbow remains in Britain until April 1912. Reasons for the extension are not that important, but could include delays with the modifications, new faults with the ship needing rectification, bureaucratic delays or government indecision on the expense, etc.
April 1912, HMCS Rainbow, no longer the first RCN vessel as HMCS Niobe preceded her, sails for Halifax with a short crew of 175, intent to embark cadets and other crew in Nova Scotia for several weeks training, and then sail around the Strait of Magellan for Esquimalt. Enroute the new Marconi wireless set breaks down, with the captain making note to order the essential components when his ship arrives in Halifax.
At 12:15 am on 15 April 1912, Rainbow is proceeding well south on the grand banks in order to avoid icebergs, when the deck watch reports a ship in apparent distress ahead. It's the RMS Titanic, dead in the water after hitting an iceberg at 11:40 pm and about to sink within 2 hours. Using his ship's spotlight and aided by Titanic's still lit decks, HMCS Rainbow's captain can see the lifeboats being prepared to launch on the davits.
What can Rainbow do to help? She's can't call for help, and you can't put all 3,500 passengers (equal to over 300 tons) onto the cruiser due to lack of space. Thankfully the sea is calm. Rainbow has just six open boats of her own, with only two of those powered.
http://www.resinshipyard.com/pages/model_gallery_pages/bcmuseum_rainbow_model.html
The first step would be for Rainbow's captain or 1st officer to transport over to Titanic to consult with Captain Smith.