I'm sorry Randy.. find another insurer. They won't insure for what reason.. fear of someone going thru the ice? You run a resort... been there.. done that with with my f-in-law... a 175 site campground.. and yes he drove a school bus in winter to make ends meet as well!! Insurance is a yearly thing.. value of the asset you want covered + liability coverage. Why would you even ask them about "what if we offered ice fishing". Just do it.. you've already paid to be covered for a year of liability from the greedy pricks.
If not... so what, do what they want.. $1100 for another Ontario Number (available through your Lawyer at said cost within 2 weeks.. mine use to keep "numbers" on the shelf so clients could have one the next day.. since been outlawed). If the resort is in your name... rent it to your spouse for the winter season... to cover your living costs pre tax and get a chequing account for the new business number.. $65 for a few hundred cheques that will last you a decade.
Water is of course an issue. Do you live there at all in the winter.. or elsewhere? Is this the main reason you don't want to make a go at the winter season.... you aren't there in the "off season" and enjoying the deep south like many resort owners do. If you are there.. you must have a water source that doesn't freeze.. tap into it. A full day with a small hoe and some 1"poly pipes.. or are the cabins uninsulated as well. I know.. it never ends.... but Adapt.. embrace... or as you've already alluded to. .. SELL.