How should be keep my cottage foundation protected from frost over winter?

Insulated concrete forms (ICF) are ideal for building foundations like yours on bedrock because you can custom-cut them to fit the unique contours of the rock. They’re also perfect in your case because they’re light and easily floated into the site by boat. I know from experience that nothing much can stop water from coming in between the rock and foundation wall if the water is there. Weeping tile can’t do it, and neither can waterproof coatings. Water will get into your crawlspace in season, drain pipes or not. The only issue is that you understand your crawlspace might be wet sometimes. Installing pipes in the lowest places around your foundation to let water drain away is the best you can do.

As for protection from frost, you won’t need any heat in your crawlspace during winter if the surrounding soil is within a foot or two of the top of the wall. My own house foundation sits directly on bedrock, and our basement has been completely unheated each winter for the last 20 years. The rock gives off enough heat to keep basement temperatures above freezing in the coldest weather, even though the basement has no insulation. The foundation wall has been fine, even though the surrounding soil is heavy, heave-prone clay. A rebar-reinforced ICF foundation will be fine without heat when built on solid bedrock.