There is a visa exemption for mariners. But if you go to China by air and then join a ship, first you need a visa to enter China.
C visa is issued to crewmembers, the requirements are as follows:

1. Valid passport (at least six months before expiration) with blank visa pages and copy of the first three pages;

2. TYPED Visa Application Form (download from this page) with one passport photo.

3. Applicants who have never been to China before should submit in person.

4. Original Visa Notice Form issued by authorized departments of Chinese Government

5. Original seaman card and copy

6. Business letters from the employing companies