We have the obvious plasters and bandages, along with stuff for bites and stings, headaches, eye wash, diarrhoea, itch cream, antiseptic cream, scissors and gawd knows what else.
If someone gets a really nasty gash/cut, don't faff about with bandages to start with. The important thing is to apply pressure and elevate. Use a clean towel to wrap around the wound. Most people haven't a clue about applying a bandage and the casualty could bleed to death whilst you try!
A couple of us asked a GP friend who sailed. He gave two brilliant bits of advice (I thought they were anyway):
First: what do you need to keep you alive and avoid permanent damage until you're rescued? Burns, breaks, bleeding mostly from memory. Can you dress wet and dry burns? Can you immobilise or strap as necessary? Can you stop bleeding from any severe wound, including head gashes? The other one was eyewash, plenty of it and a good method of delivering it (not eye-baths apparently - they can suck the eyeball out!!). That's your first aid kit.
Second: What do you need to stop your weekend being ruined. Go and get the contents of your first aid drawer at home and empty it on the table. Make two piles - stuff you've used more than once in the last year and stuff you haven't. Go out and get the most powerful versions of the stuff in the first pile you can find and keep it as your first aid drawer on the boat. (He also told us to go through both piles and throw out everything that's out of date but that's another matter).
He got his stuff from St John's Ambulance (http://www.stjohnsupplies.co.uk/ although one or two looked suspiciously like dressings I've seen in A&E...
From a personal point of view my other one is don't forget any personal medication (including the crew's). If you have a grab bag put some in there as well.