Replying to Waterproofing.......nescessary?

me and my friends just built a 6 foot drop in with a 10 ft ramp that we are gonna cover with astroturf for the summer.

when i showed the setup to some other people the first thing they said was

"o man that is gonna rot unless you coated it".......which we didnt.

so before we cover it with astroturf is it absolutely necessary to waterproof it?