Basic engine repair skills, something to patch up your boat in case you spring a leak, a basic comms kit+genny so you can call up the coast guard or a passing boat if you should need them (assuming your island is out of cell phone reach), cache of freshwater, fuel, waterproofed matches and a couple of days worth of tin cans in case you ever get cut off. Halifax coast in winter can be, how to phrase this, impressive, to say the least.
Mind you, the real kick is when you are isolated but there is an additional degree of risk there. No need to get your appendix out as a precaution just yet ;)
All of this is assuming you're on the salty side of things, if you're in a lake it's a lot more relaxed but you will still need to take basic precautions and you'll need to be extremely careful with what you do with your wastes (you'd need to be just as careful in the ocean with that for ethical reasons but there will be fewer watching eyes so you can drop the 'extremely').
Maybe you could make a weekend of it for your families or something.
Non-nonsense survival guide:
http://www.amazon.com/gp/aw/d/0762448180
Also: man vs wild (television)
I would very strongly advise AGAINST emulating Bear Grylls' antics in 'Man vs Wild'. He often does things that are the complete and total opposite of what you should do. One example that stuck in my mind was when he (supposedly) needed to cross a mountain and decided, instead, to go through the mountain via a cave, which is possibly the most stupid idea I've ever heard. He arguably puts young people at risk by encouraging such dangerous behaviour. Lofty Wiseman (author of the SAS Survival Guide) has described him as an "idiot".
Also, while the TV show portrays him as roughing it in the wild, the truth is somewhat different: http://www.dailymail.co.uk/news/article-470155/How-Bear-Gryl...
In terms of winter storms, this is approximately as bad as it gets: http://en.wikipedia.org/wiki/White_Juan .
https://maps.google.com/maps?q=cap+chat+quebec&oe=utf-8&aq=t...
The st. Lawrence river might just as well be ocean for the purpose of driving along the shore road in winter, in the middle of a snowstorm like that it really makes you wonder what the hell you were thinking of driving there in winter.
I went there to visit a huge VAT ('eggbeater' or Darrieus rotor windmill), I should have kept a better eye out for the weather reports rather than to keep on driving, and I only caught the tail end of it. Live & learn...
(someone else's picture of the windmill: http://www.flickr.com/photos/libraryman_76021/555075211/ )
It's an interesting experience to try to dig out your car before the next snowplow comes along.
https://maps.google.com/maps?q=cap+chat+quebec&hl=en&ie=UTF8...
Yep, it's the 4th most powerful known meteor impact: http://en.wikipedia.org/wiki/Ren%C3%A9-Levasseur_Island
I love Wikipedia.
http://www.outdoored.com/community/risk_management/b/wildmed...
And an AED