Absolutely understand your question and concern cuz' dem bu'bs ain't cheap! Plus, I thought I'd cook my ladies with a 250W heat lamp but couldn't find red in any less wattage...
All that considered: It depends on the ambient temp around your brooder. I have 15 chicks, and when they were in a 15"x36"x18"h brooder inside the house, a 100W red flood light easily maintained 80 to 85 degrees depending on how close I had the light, and it's normally 65 to 70 degrees inside.
When I had to move them to the outside brooder (4'x4'x2'h) at 4 weeks old, I had to put a 250W heat lamp on 'em because our temps still get into the 30's at night and they aren't entirely feathered out. The 250W heat lamp keeps one end of the brooder at 75 or slightly less, and the ladies are quite comfy, while the 100W red flood didn't do squat... (60* or less in the daytime...)
MORE importantly! Don't use a standard work lamp assembly, because they aren't designed to work 24/7. Be sure the lamp itself has the ceramic socket and is designed to handle the heat and continuous use.
Just my MINIMAL experience! Good Luck!
Whoops! Forgot the water question... My brooder is in a covered run and I have ply against the wire to prevent rain splash. Flood light bulbs are designed to tolerate weather, but unless the bulb is in an outdoor rated fixture, I don't trust water and electricity, EVER!