You will be fine. Just get it out as soon as you can - it needs to drop in the teens for a good 8 hours for any damage to occur. First the cold needs to combat the residual heat from the sun during the day to get the water in the intercooler to the point of freezing. Not sure about keeping the lower unit in the water unless the outboard is self draining. I know a lot of OBs claim they are self draining but there is still a lot of water trapped in the head. The old optimax's were terrible if not completely drained - the water cooled the fuel pump, air compressor, head and electronics.