Drain it. Determine if water was over trans fill. If it was, used a vacuum removal and suck off the bottom. Water is heavier and will fall to the bottom. Check the fluid removed for water, if no water, refill and no worries. If there is, don't know the best sure fire way to removed all the water.
Let it dry but there will be little near term problem with starter or alt if they were covered. Rain water is "better" than dirty lake water or salt for sure. Life will likely be shortened but could be 1-4 yrs before it needs to be rebuilt.
For the nubies worried about block freezing - all but zero chance. General rule is 24 degrees for 24 hours before damage. That means, temp does NOT get above 24 degrees for more than a day!
A casual dip overnight will have zero effect. Set out a cup of water, likely it will not even freeze over let alone solid, now consider your block is insulated in the engine compartment and has 500# of metal that also has to cool below freezing before water can freeze.
1991 MariStar 240sc, 454, tower, WetSounds, Krypt and Rockford Fosgate