It looks like those are the waterside pieces of the four-person family raft slide that didn't get built:
Sources: https://m.weibo.cn/2733921017/4648024133604616 and https://m.weibo.cn/1771522350/4648846817236022 and https://m.weibo.cn/5653289328/4648684400414132 and https://m.weibo.cn/2886760110/4650203405354237 and https://m.weibo.cn/2077432315/4649858864251315 and https://m.weibo.cn/1634614497/4650290814913700