Not wishing to pre-empt anything, but...
"...temperature 21 degrees Celsius, dew point 19 degrees Celsius..."
...assuming this was not a fuel injected Raven II, that's real carb-ice territory.
Add over-water operation and less than full load operation, ie: partial throttle settings, and you're there. It only takes a momentary lack of sufficient carb heat and...
Hope I'm wrong