I used to teach rescue diving
and diver first aid, and I hesitate to provide a list of products because without training you may misunderstand the list. For example, if I list "aspirin" you might think this is a remedy for headaches, while I intend it as something to administer during a heart attack. Not knowing your level of training, my preference is to discuss situations. Should I mention "heart attack," properly trained persons might want to equip themselves with oxygen; however, oxygen would not be an appropriate item for your first aid kit without the training. One last example: I'd never venture offshore
without a scalpel, because I'm able to use it to save your life in a few situations. However, I'm not recommending it for your kit for obvious reasons. Also, there are various ways to deal with every situation. Take for example a jellyfish sting. On one hand, meat tenderizer is effective to deal with the venom, which after all is a protein. On the other hand, there are medicinal remedies that might be more effective, as per your pharmacist's recommendations. With this disclaimer, then, here are some
of the situations you should be equipped to handle:
-allergic reactions, (including anaphlylaxis if someone on your crew is sensitive to things like bees or peanuts)
-swimmer's ear and wax buildup (if you're divers)
-nasal congestion/cold symptoms
-skin grunge and minor lacerations
Additionally, consider tools such as: thermometer, tweezers, bandage scissors, CPR mask, latex gloves, et cetera. Also a good idea to have a bunch of hand sanitizer, et cetera.
Finally, I agree with Mark, the best thing you can do is get some training. First Responder, Wilderness First Aid, and Diver First Aid are all great ideas. At the very minimum, get CPR certification
One last caveat, I'd recommend a more complete kit for offshore passages.