Invariably, whatever spares and tools you carry the Law of Murphy states that you will not have the one you need when things go wrong!!! Either that or you will carry so much extra weight that the boat will sink!!!
Seriously though, whilst spare keys, kill-cord and fuses are a must, I am a great believer in the principle that if I regularly check and service my boat and equipment it is most unlikely to let me down, The majority of incidents are caused by contaminated fuel or electrical terminal corrosion/failure, as a consequence, these areas should get careful attention. I also contend that large tool kits are of limited use, especially as most modern outboards are incredibly reliable, and if anything does go wrong they are not the simple mechanical units we played around with less than 10 years ago. I look at my modern 4-stroke and consider it is way beyond my capabilities even though I consider myself to be a fairly competent DIY engineer.
Finally, the most useful tool in my kit is a compact Multi-Tool, it is a LeatherMan Juice KF4, small enough to fit in my pocket but very useful.