Here's a link that shows you what you should carry, made up by a
cruising doctor: http://www.riparia.org/cruising_medical_kit.html
Even if you don't want to build your own, it's a good reference to
compare the commercially available ones. Also look at:
http://www.equipped.org/medical.htm#MedicalKits
http://www.landfallnav.com/medicalequip.html
http://www.marmed.com/kits.shtml