Cold water immersion makes a difference, but the season probably doesn't matter as much as you may think. Many Canadian lakes are cold throughout the year anyway.
There are a bunch of resource you can consult.
Red Cross Canada
- no life vest on (PFD)
Fatalities in Canada, 2008–2017
The temperature (page 12) doesn't figure highly, but... keep in mind people are more out in summer, so an equivalent number of deaths in cold weather likely means more risk (as your question presupposes).
Risk Factors for Recreational Boating Deaths in Canada
again, alcohol, no PFD.
Another resource I found had a cause of death contribution factor and water temperature was only listed as a principal factor about 20% of the time IIRC.
Ah, found another one by Red Cross, the cheerily titled BOATING
Immersion and Trauma
Deaths in Canada
Table 11b, page 45. There is certainly a correlation, especially considering that fewer people will be out in cold weather.
From a quick look at the map, this is a small lake so it should be fairly calm and easy to get back to shore if the wind rises.
On the other hand, I own a book called Outdoor Survival in British Columbia. The author is down to earth about most risks such as bears, cougars, lightning, etc...
He reserves his strongest warning about risk of death when being out on a cold lake unprepared.
Bottom line is that you would gain more safety from insisting on some ground rules when you do go out fishing with these people any time of the year than paying too much attention to the exact season.
No alcohol beforehand or during for you or anyone on the boat. WEAR a PFD (must be on for it to work, having it in the boat doesn't count if you can't access it while in the water). Make sure the boat is in good shape and reasonably stable, enough so that it won't tip over if one person falls in and the next person screws up bringing them back onboard. Talk through what you would do if someone fell in. Make sure no one has any medical risks worsened by cold water. Do you trust these people?
That all said, as long as you are careful, this doesn't look much like a high risk outing. Don't let yourself feel pressured, but enjoy it if you do decide to go through with it.