With ubiquitous electronic communication, it is not really common anymore to send letters or post cards to ship crew, but parcels are still adressable and probably still not too uncommon for birthdays or other holidays if you have a loved one working on a ship. If parcels arrive, I would very much assume that it is still possible to use the same way for letters.
The common procedure, but shipping companies may for sure have different policies on how to handle mail adressed to their crew, is to send the parcel (or letter) to an upcoming port and use an address similar to:
Ship name
Attention of: crew member name
Port's full address
Be aware that international delivery of both letters and parcels may take quite some time, so make sure that you plan plenty of time for the delivery to arrive at the port. If you send a parcel, it should contain nothing with relevance for custom's clearance to make sure that it is delivered to the port without any custom processing.