There are a few professional recommendations available:
I like the
small book by Eric Weiss that comes with a certain brand of grotesquely over-priced first aid kits but is also available separately. Oddly enough, the kits do not contain the items recommended by Dr. Weiss. IIRC, the book has almost a flow-chart approach to dealing with emergencies. May require reading glasses for the fine print.
At the other end of the spectrum are the comprehensive
WHO Medical Guide for Ships and UK
Ships Captains Medical Guide, both of which are free on-line but may be overkill for a small pleasure boat. But maybe not. Oh, it looks like the newest version of SCMG is no longer free, but PDFs of the
previous edition can still be found.
The price of the "marine first aid kits" that one can buy far exceeds the cost of their contents, purchased separately. (Still bitter about buying one...) But you can download the contents list, for some of them.
e.g. Although shopping for all of that stuff could take a significant amount of time and effort, which the kit saves you. And the one that I bought did not actually contain all the things that it claimed to.
A few random things I noted when I stocked up - and/or discovered when I had to use it:
Things like gauze bandages and tape can be bulky and kits are likely to contain only one (1) of each. However, if you need to treat a significant wound for several days, you will need several. They aren't expensive, but do take up a lot of space.
Wound closure strips work great, compared with trying to stitch yourself - but you will need the tincture of benzoin needed to make them stick. (Not included in kit.) And probably more than one packet because once opened, they are no longer sterile. (Same caveat for everything else wound-related - individually-wrapped better than bulk-pack.)
Almost a joke: the "six person kit" contained one (1) dose of seasickness medication. I imagine sick people fighting over who gets the one dose...
Speaking of medications, they were all packaged in little foil packets with text printed in tiny pastel print. Practically unreadable to (ahem) "mature eyes." And no explicit labeling as to what each item does or what condition it is for. When I've purchased new stuff, I've added large print labels with the name, usage, and dose.
Most medical thermometers that one can buy only cover a tiny range - like 96 - 103 degrees. But mariners might need one that covers a lower range - for hypothermia. Takes some searching to find one.