I guess you're right on the walls part, even a 5-10mph ding with that barrier will make a decent sized dent in your car. Just stay in the middle of the track, don't be a "super sick skid missile hero" and you'll be fine.
But other than that the wet track makes it much easier to initiate and slows the speed right down, I found it gives you way more time to get a feel for the car, how your input affects it's position on the track and so on. I went there with a £500 318is a couple of weeks ago and had non stop seat time all day, exactly what you need for learning!