I don't carry much, since I run a fairly new motor, do lotsa shore maintenance, and only do daily inland trips (no more than 60 miles, and never more than ten miles from a ramp). I do carry a spare prop, hub kit w/ nut and a breaker bar/socket for the prop. A small socket set or nut drivers to fit the t-stat housing. I carry a needlenose leatherman to do lotsa small jobs (including pulling hard to reach fuses). A roll of duct tape and a roll of electrical tape and some zip ties. In extreme cases, I may carry an old cell phone and charger (even without service, it can be used to dial 911). I do keep one of those portable power supplies (10,000mAh for around $30, about the size of a smartphone and will recharge a smartphone about 4-5x) in my lunchbox. Thinking about getting one of those MicroStart XP1 units for a main battery backup...
I'm surprised none of you off-shore guys have mentioned a wax toilet ring. They are cheap (less than $5) and work well for plugging holes in the boat.