The more weight in the raft the more momentum the raft has when facing the wind. There's the part of the slide where it dips down then shoots up a hill, assisted by the water jets. That portion of the slide is not completely enclosed. A rider can get stuck if there isn't enough momentum to get up the hill. If a wind is blowing against the raft, it needs more momentum than normal to get up the hill. Think of throwing something against the wind. If you throw a rock into the wind, it might not go as fast or as far as if there were no wind, but it's still going to get pretty far. If you toss a cotton ball into the wind, it's not going to go far and might end up behind you because there's not enough mass to keep the forward motion against the wind. Same with Aquaduck riders, facing a wind, there needs to be more mass on the raft to keep the raft moving forward.