Put your carb and crankcase vent hoses (not the float bowl overflow drain tube) up through the bottom drain opening in your air box and then seal around them and close and seal all the air box openings except the top to keep water out of your engine.
I did my second enduro last weekend, first in 15 years and my first in Florida, a short 25 mile family Enduro. I didn't water proof my bike. I made it a mile and a half then sucked water in to my engine. There were about a half a dozen of us that made it less than 4 miles. Several hours later, with help, I was able to push my bike back to the truck.
Remember, this is Florida, there is a lot of low land, and it rained last week, you will get wet.
I think my boots are still wet.