I would suggest reading up on the laws in your state (and any state that you boat in) as some have different rules. For instance I just discovered last year that IL requires you to carry a whistle that can be heard for at least 1/2 mile. I'm not sure if its required in all states, but many require a throwable flotation device SEPARATE from the individual PFDs needed for each person on the boat.

Other than the laws and what others have mentioned, the only thing I would add is a fire extinguisher.