I don't think it was the hot day and cold water. The seals around those front headlights often go bad. Mine did the same thing. Water and condensation stayed in it for weeks until I took it in to the dealer. In fact the glass often fades a dirty gray color which some people supposedly grind down with light sandpaper then buff it out. As for me I just bought a new, very expensive replacement. But when you are trying to keep your car at it's best it's worth it. I replaced one headlight because of your issue with the seal and another one because of the fading problem.
There are a few different options. Depending on the age and condition of your light, you may want to consider replacing it.
If thats not an option, you can look into a few different options:
1) Park the car in the sun and let it evaporate. This will take a lot of time, but the water will evaporate over time depending how much went inside. Some people add sealant around the outer seam of the light, this is a cheap and simple solution.
2) You can disassemble and re-seal, then re-assemble. This is very time consuming and many hobbyists make mistakes that effect the integrity of the light. You can warp the light, use the wrong sealant, or cause other damage - I've seen endless amounts of costly mistakes.
3) Some folks have drilled numerous holes on the bottom of the light. This works, but I wouldnt recommend this as debri can enter the light.
We published an interesting article about this just recently, makes good reading for you folks with older headlights:
http://clearcorners.com/products/004/
good luck!
cheers
mike / clearcorners.com