The east side of the island has beaches and is lower towards sea level ..and a hundred people tops would be dislocated due to rise of sea level ...
But the rest of the island is 300 to 500 feet above.
Once off the ferry it is a climb for everyone up the roads...
The island is pretty much vertical and plateaus ...
We wouldn't lose much of the island from sea level rise...

Me...my place is about 300 feet above sea level..
I am in no threat.
Likewise most of the population here...
Also with a population of 4000+/- ...I would imagine lookout posts and shifts of lookouts would be posted to available approaches if the crap hit the fan that seriously.

Also...The harbours at Nanaimo and up and down the coast of Van Isle would empty rather quick with the beginning collapse of society...
all the boat owners would be running to secure their boats, or get away ..or sail them to secure out of the way locations...
First thing that would happen is many would try to flee the islands ...thinking it their only chance just to get back to the mainland...
After the initial dust settles..I think there wouldn't be a boat at dock anywhere around...

The bigger threat would be from the mainland later on...
With boats coming to look to take food and provisions...
Yet again though...
By the time that kind of planned venture was organized I would expect that few would be around to properly sail the vessels, and those that were proper mariners would be long gone...
and I would guess without a civil society , Marine AIS and satellite tracking and all those easy easy methods of avoiding grounding, going up on rocks and other hazards locally, won't be available...
The currents are swift and rough around Gabriola...
and even with all kinds of tracking equipment...I see so many ships sink, grounded, capsized, etc ...just in the harbour and around Gabriola ...as it is ...every year...
So only the foolish and desperate would try... post civil society collapse.

Just read from the weather network...
2019 could be the hottest year in human history...
not good.