How far in advance can you buy Shenzhen-HK ferry tickets?

Hello! I will apply for an L tourist visa in order to visit Shenzhen, and need to provide proof of a travel ticket to leave Mainland China. I plan to do that by crossing the border into Hong Kong. It seems that train tickets can only be booked 15 days in advance, so I couldn’t provide the visa authorities with a train ticket as proof.

Is it possible to purchase ferry tickets well in advance? Or perhaps coach tickets might work?

Thanks for any advice!