Knowing when to de-winterize your boat and put it into the water is important so you don’t accidentally damage your boat. If your boat is not winterized or kept in a warm garage, any temperature below freezing that lasts 1.5-3+ days can cause damage such as a cracked engine block.
It’s best to put your boat in the first week there are no freezing days projected on the 10-day forecast. If the temperature drops slightly below freezing for a few hours at night it will be fine, but I always recommend being patient.
For many people in the USA, this is around the first few weeks of April. However, make sure you are always checking the forecast to be prepared in case of a sudden cold front.
If you are worried about engine damage from freezing, consider placing a small heating lamp or an engine compartment heater inside your engine bay (links go to products on Amazon). You could also drain any water out of your engine by loosening the engine water drainage plug or loosening the water pump (for inboard engines). Outboard engines will self-drain as long as they are fully upright.
Assuming that the water is not frozen, it would be best to keep your boat in the water during freezing air temperatures. This is because if there is no ice, the water is presumptively warmer than the air. However, if your boat is in saltwater, it may be a different story as saltwater freezes at a lower temperature. I would recommend testing the water temperature off saltwater and deciding based on that temp. reading.
As long as you put an outboard engine fully upright for a few minutes, almost all of the water will drain out of it. Inboards require you to manually drain the water through either a drainage plug or by loosening the actual water pump under the engine block. So, based on the fact that it’s a bigger pain in the butt to drain the water from an inboard engine, I would say you need to worry more about it freezing.
Ice can crack or damage hulls very easily, so if you plan on leaving your boat in freezing water, make sure to have a de-icing system and close any thru-hulls below the water line. You’ll also need to winterize your engine properly or at least drain out all of the water if it’s only going to be there for a short amount of time.