Shenzhen Short Term Visit

Dear Sirs,

in case of entrance to Shenzhen via ferry route from Hong Kong, with less than 3 full days stay and travel back to Hong Kong, is visa free entrance for such short stay safe option ?
Also, as evidence, is it enough to provide an invitation from Chines business partner and flight tickets or something else (any form to fill-in in advance)?
Thank you, Ivo from Split