While a lot of people will tell you this or that US Coast Guard required piece of equipment, I would say that the best thing to have on-board is an approved boating safety course. BoatUS has an on-line course, that is excellent, USCG approved, and it saved me $75 per year on my insurance!
Free Official Online Boating Safety Course From The BoatUS Foundation
Recreational boats must carry Coast Guard approved Personal Flotation Devices, in good and serviceable condition, and of the appropriate size for the intended user. Wearable PFDs must be readily accessible, not stowed in bags, locked or closed compartments or have other gear stowed on top of them. Throwable devices must be immediately available for use. There must be one Type I, II, III, or V PFD for each person on board or being towed on water skis, etc., PLUS one Type IV throwable device. Throwable, Type IV PFDs may no longer be substituted for wearable types on boats less than 16 feet. State laws on mandatory PFD wear may vary.
Must carry approved visual distress signals for nighttime use.
Every vessel less that 39.4 feet (12 meters) long must carry an efficient sound-producing device.
According to the US Coast Guard Auxillary Safety Inspector, AND my boating safety course, sitting on it destroys the buoyancy of it. If they see you sitting on it, they (theoretically) can write you up for not having a properly maintained Class IV.