There were no military actions on the China station 1901-1904. The medal awarded in 1903 could be for the Boxer Rebellion 1900 (3rd China War if you prefer) or it could be a Long Service & Good Conduct medal (LSGC) or something similar. What ship was he serving on in 1900 and what's the context for the entry about the medal?
1901 census will show Marines only if in England & Wales or aboard ship at home. If abroad, then don't bother looking for them basically. I assume by Medal Cards you mean the Medal Index Cards online? They are for the Army only and for WW1 only, so not relevant, sorry.
For the history, campaigns and military actions involving the Royal Marines, Google will be your friend to begin with along with library books and the like! It's a generalisation of course, but because Royal Marines served aboard RN ships, you can assume Marines were involved one way or another in the same campaigns as the Royal Navy. So that should also help you search