We're sailing on Georgian Bay where things are always still quite cool in May and early June, (with temperatures below freezing overnight in early May) but our season is short so we try and get started early. When we purchased our Catalina 36MkII we also got Espar diesel heating installed to help extend our season. Great idea, but not so good in reality.
The problem is condensation, with everything dripping in no time flat, from the un-insulated ceiling and especially the (metal frame) hatches.
I do not believe that there is a real solution to this problem. It would take quite bit of work to do a good job installing a false ceiling, which would not get rid of the insulation from the hatches. However, I am putting this out here for comment in case you folks out there have a smart idea about this!