Due to the large presence of the Law Enforcement on the water doing safety checks lately, I figured it would be a good idea to put together a list of items they are looking for. This will help you avoid getting cited for items and having to pay fines - and more importantly, be safe. Alan (aka Boat Cop on PB) works for the Lapaz County Sheriff and assisted me putting together a list of items.
Be Sober - I think this is self explanatory
Registration - Make sure your registration is current and you have your certificate of numbers (registration card) with you on the boat
Numbers properly displayed – Block letters, minimum 3” in height, contrasting color to the hull, visible from 50’ away, and properly spaced: AZ 1234 AB or AZ-1234-AB. Current annual decal displayed.
Life Jackets - One Type I, II, III, or V wearable PFD for each person on board. For a type V to be counted, it must be worn. (must be USCG approved, readily accessible and in good, serviceable condition). Also, if the boat is over 16 feet in length, you must carry One Type IV (throwable) PFD. The Type IV throwable must be immediately available. Children 12 years old and under must wear their PFD when the boat is underway. (Underway means not moored, anchored or aground. Drifting with the engine off IS underway)
Example of Type IV Throwable PFDs

Visual Distress signals – Must be carried on Coastal Waters (Ocean) and the Great Lakes. Not required, but recommended, on inland waters.
Fire Extinguisher – Boats less than 26 ft have to have 1 class B1 hand held F/E. 26 ft to less than 40 ft – 2 B1 F/E or 1 B2 F/E. 40 ft to 65 ft 3 BI F/Es or 1 B2 & 1 B1. Fire Extinguishers must be USCG Approved, fully charged, in good condition and readily accessible. A fixed system in the engine compartment can take the place of 1 B1 F/E regardless of size. (Arizona requires that boats less than 26 ft carry a hand held B1 even if equipped with a fixed system)
Sound Producing Device - A vessel over 39.4 ft (12 meters) must, at a minimum, have some means of making an "efficient" sound signal - (i.e. handheld air horn, powered horn, etc. - Human voice/sound not acceptable). Must be audible for at least ½ mile. Boats under 39.4 feet don’t have to carry a horn or whistle, but are required to give proper sound signals, when required.
Ventilation - All vessels with enclosed engine must have natural ventilation (at least two ducts fitted with cowls). All vessels built after 31 July 1980 must have rated power exhaust blower.
Backfire Flame Arrestor - Required on gasoline engines
Navigation Lights - Required to be displayed from sunset to sunrise and in or near areas of reduced visibility. In California, Personal Watercraft cannot be operated between sunset and sunrise whether they have lights or not
Muffling devices – Each boat must be equipped with an effective means to muffle the exhaust noise. The device(s) must be in constant operation and reduce noise to certain levels
Ski flag – To be displayed if there is a skier (or rope, in California) down in the water.
Ski Observer – Must be 12 years of age or older and constantly observing the skier.
CA: A person under 16 may not operate any vessel greater than 8 HP alone. 12 to 15 may operate with an adult actually on board the vessel. Under 12 may not operate.
AZ: A person under 12 may not operate any vessel greater than 8 HP alone. Under 12 may operate with an adult actually on board the vessel.
The Colorado River and its lakes are dual jurisdiction waterways, where each state (AZ, CA, NV, UT) may enforce their own laws anywhere on the River or lake. You may be required to comply with the most restrictive law, when there are differences in regulations (ie age of operation)
Check local laws and regulations wherever you boat, as they may differ from these. Taking a safe boating course will help you learn and understand the law and safe boating practices.