Thanks for the insight Tburdock. Not sure where in Nova Scotia you are, but the temperature is somewhat affected by the proximity of the Atlantic I believe. The average daily low in the coldest month in Halifax is -9C. In Montreal it is -15C. (That's 16 vs 5 Farenheit for our US neighbors.) Hopefully that extra foot depth is enough. I suppose I could go deeper if really needed, but unless I decide to keep Koi I'd prefer 4' - shallow enough not to freak out the non-swimmers here too much ('cause you know they'll fall in!).
I checked with the insurance co and the town today. Surprisingly the insurance folks don't have any requirements, not even a fence! It's like I caught them by surprise ("hmm, we didn't think of that!"). The town say no fence needed for ponds 18" deep or less (although they recommend a 4' fence and I would put one just the same). 18" deep just isn't practical though - I coud only keep the number and size of fish that I would be able to overwinter indoors, or re-stock each year (not my style). Plus I would have to think racoons, birds and such could be a problem in 18" given than the top of a big fat fish may only be 12" down when resting off the bottom.
So swimming pool rules apply - 6' fence required. Mucho dinero. It'll be awhile...