This is Water Slide map mod for Grand Theft Auto IV. Slide have 23 objects and it's located in Alderney. You can use this with your friends or just alone in SinglePlayer.
Press M+Num 4 to start Ragdoll, then just use WASD to control your player. I tested this with my friends and they said that they see this so your friends see this too, but if you wanna be sure you can give this to your friend.