Be aware that first aid offshore is pretty different from onshore. If you serious about it, start with downloading the pdf files from here:
Ships Captain's Medical Guide
Its not sufficient, but you will find how to continue.
Some useful reading: Ditch medicine by Hugh L. Coffee, looks heavy, but you might find the situations which you will (hopefully never) experience.
Know how to reach Medical Advice/Assistance (VHF/HF), think about with whom the MRCC should connect you (Singapore, Rome, etc. might be better than others, collect this information before you need it)
Have the proper form filled out BEFORE you call the doctor (you might download one before you start) otherwise you might run up and down the a lot before you are able to answer the questions.
If you just want to do some coastal cruising, forget about the above, except the useful frequenices, maybe. -)