Ferry to Hongkong from Hainan Island

Hi,
So what about this ferry from Island to Hongkong? They exist? I wanted to spend some time on the Island and then return home by plane, departing from HK. I wanted to spend as little money as possible, so I would rather don't use plane to get back to HK. I read a little and I know I could take train Haikou->Shenzen->HK, but it would take at least 13 hours I guess. So.. what about those ferries? Thanks for help.