Ron:
If you are on inland waters a flare gun is not necessary. If you go out into the ocean, the Great Lakes or any of the large bays then a flare gun would be a good idea. On a lake or river I would consider them to be more of a hazard than a help (think kids, or your drunken friends).
The legal requirements vary from state to state, but most common is:
Fire Extinguisher: The laws usualy specify that it should be mounted someplace (verses kicking around in a locker) but I have never been hassled about that.
Life Jackets: At least one, of appropriate size (adult jackets won't work for a child) for every passenger. There is often some obscure wording that the law enforcement can interpret as the life jackets have to be immediatly available, not buried back behind the sub woofer. They also need to be in good shape, no tears in the fabric, etc. If they have been rotting in the bilge for a few years you should replace them. The cheap orange ones are actually the best.
Throwable device: This is a "type IV" device, the ring or the "cushion". Again, wording of the law can be interpreted that it has to be "immediatly available", not buried. My suggestion: have two. One still in the plastic wrapper put away out of harms reach. The second one sits out. Use it for a fender, an ice chest cushion, fooling around in the water, etc. If yo use it a lot it will be handy. If the officer complains that it has been used as a fender too much pull out the pristine one.
Sound Signaling device: it can be the electric horn, one of those air in a can horns, or even a whistle. I got hassled once because my whistle was on the key ring and I had to shut the engine off in order to blow the whistle.....
At night you need lights. Don't need them during the day. Requirements vary with boat size, but for our discussion it is basically the red/green bow lights and an all around white light.
Paddle: probably varies from state to state and will also depend on boat size. For California, boats under 16 feet are supposed to carry a paddle. I have a nice paddle that has a collaspable handle. Stores really easy, and has actually come in handy a couple of times.
If you are pulling a skier you may need the "skier down" flag. This varies state to state.
In addition to the legal requirements there are a bunch more recommendations.
The Coast Guard Auxiliary (which is a civilian group, NOT a law enforcement group) has a "Courtesy Marine Examination" program where they check the boat for their list of items and will apply a CME decal if the boat passes (no ramifications if you fail). Here is a website that details their requirements:
http://swansongrp.com/cme.html For our discussion, the basic additions are a "detwatering device" (bilge pump) and an anchor, both of which I highly recommend.
The bilge pump should be obvious. The bigger the better. Make it two!
An anchor can really save your ass. Or at least save your hull. If your engine fails you will be at the mercy of the wind. The wind will eventually blow you against a shore. If it is sand and the wind is light you might be okay. If it rock or the wind is strong your boat can be pounded to death. Being able to throw and anchor and wait for rescue can save the boat.