Not being familiar with the brand I would suggest checking the specs on the can or website for temperature tolerances.

  Putting a couple of lights inside the boat will raise the hull temp considerably too. I've noticed cooler temps sometimes come with lower humidity - keeps one pretty busy maintaining a wet edge.

  One benefit is less bugs in your shine, my Wood Duck is a floating museum!

I varnished my second Guillemot with Epifanes two component Polyurethane varnish. Temperatures were about 0 to 5 degrees celsius. The result is not good - a lot of air bubbles remained inside of the varnis layer.

The main problem ist the very fast hardening of the varnish surface in combination with the high viscosity of the varnish at low temperatures. This prevents nearby all air bubbles (there will always be some of them) to reach the layer surface where they can be removed with a foam brush with ease.

I've sanded the boat, and am waiting now for higher temperatures (about 15 - 20 degress celsius is o.k. for this kind of varnish - as shown on the can :-)).

My first Guillemot was varnished at this temperatures - no problems with air bubbles at all ...

And yes, I know how to use this varnish.

A potential solution to this problem could be to spray the varnish - under the assumption that you are able to do it clean and without any runs. And this technique wastes about 1/2 or more of the varnish that will not hit your boat. Professional sprayers say, that it could be even more. And if the temperature is too low, the varnish will not flow properly, and the surface will not be really glossy, but somehow rough.