Lots of good suggestions I usually opt for a battery pack charger instead of the jumper cables, but I do usually have both.
I also like to have a tool kit, one of those all inclusive kits you can get for around $50, they have a little bit of everything in them.
In a large part it depends where I’m boating, normally I’m not going to be too far from houses, other boats or resorts. My biggest area of concern is always on the river in winter/spring, since I know my TM wouldn’t do much good. to get myself up river.