Most packing lists are opinions. Part of this one is not: some of what you carry is required by federal law, and the requirement is not only that you own it.
The rules below are the federal floor, quoted from the Coast Guard's own guide. States add their own, and they differ, so check yours rather than treating this as the whole answer. The specifics by boat length are in the document linked at the foot of this page.
Life jackets: one each, and one to throw
"All recreational vessels must carry one wearable PFD for each person on board. Any boat 16 feet and longer (except canoes and kayaks) must also carry one throwable PFD."
Four conditions have to hold. A PFD must be "in good and serviceable condition", "appropriate size and type for the intended user", "appropriate for the intended activity", and "U.S. Coast Guard-approved (check the label)."
Worth reading that second one twice if you carry guests. An adult jacket does not satisfy the requirement for a child aboard, and child jackets are approved for specific weight ranges.
Where it is stowed is part of the requirement
This is the part people fail, and it fails quietly, because the gear is on the boat and looks like compliance.
The Coast Guard is specific. Wearable PFDs "must be readily accessible" — you should be able to put them on in a reasonable amount of time in an emergency — and "should not be stowed in plastic bags, in locked or closed compartments, or have other gear stowed on top of them."
Throwable PFDs "must be immediately available for use", and "should be on the main deck within arm's reach, hanging on a lifeline, or other easily reached location."
A life jacket still in its wrapper under the cooler is not readily accessible. Neither is one in a locker you have to move two rods and a tackle bag to open. If you do one thing after reading this, go and look at where yours actually are.
Children under 13
"On a vessel that is underway, children under 13 years of age must wear an appropriate U.S. Coast Guard-approved life jacket unless they are: below deck, or within an enclosed cabin."
And the caveat that catches people who boat across a state line: "If a State has established a child life jacket wear requirement for children that differs from the Coast Guard requirement, the State's requirement will be applicable on waters subject to that State's jurisdiction."
Signals: check the date before you leave, not offshore
Smaller boats get a partial exemption from day signals but not from night ones. Recreational boats under 16 feet, manually propelled boats, open sailboats under 26 feet without propulsion machinery, and boats in organised events "are not required to carry day signals, but must carry night signals when operating from sunset to sunrise."
Pyrotechnic signals expire, and that is a packing-day check rather than an on-the-water discovery: they "must be U.S. Coast Guard-approved, not expired, in serviceable condition, and readily accessible", with an expiration date 42 months after manufacture.
The Coast Guard also makes a point worth taking on its own terms: "Although the VDS carriage requirements are only enforceable in areas where VDS are required by law, signaling devices are recommended wherever you operate your boat."
Sound, and two things that cost nothing
Sound signals are not only for fog: "Recreational vessels are also required to use sound signals during periods of reduced visibility and while at anchor."
And the cheapest additions on the list are ones the Coast Guard names as optional: "Some items that are not required but are a good idea to have with your wearable PFD are a whistle and an emergency light." Both attach to the jacket, so they are where you are, not where the boat is.
Before the gear, two minutes at the kitchen table
Packing is the last step, not the first. Check what the water and wind are doing on the trip planner, and leave a float plan with someone ashore — the float plan guide covers what it needs to say and who should hold it. Neither weighs anything, and between them they cover the two failures that gear cannot: going out into conditions you would have declined, and nobody knowing where to look.