The best place to start is the gear you are legally required to carry, for two reasons: it is the law, and it is also the gear most likely to save your life. The US Coast Guard sets a federal minimum that scales with boat length, and a 20 to 45 ft coastal boat sits in the middle of that scale. The table below is the federal floor; your state can require more, so check with your state boating authority, and treat the minimum as a floor, not a target.
Two recent changes catch owners out. Since April 20, 2022, disposable (non-rechargeable) fire extinguishers carry a twelve-year service life from the date stamped on the bottle, and boats with a model year of 2018 or newer must carry the newer 5-B, 10-B, or 20-B labeled extinguishers rather than the old B-I and B-II; a 10-B counts the same as a 5-B wherever the table lists one. And since April 2021, the operator of a boat under 26 ft with an engine capable of 115 pounds of static thrust, roughly 3 horsepower, must use the engine cut-off switch link when the boat is on plane or above displacement speed, unless the primary helm sits inside an enclosed cabin. That use rule bites once the boat has a switch installed, which has been required on new boats built since January 2020. One thing the Coast Guard does not require: AIS. It is not mandatory on any recreational boat, but as the navigation section explains, owners near commercial traffic should still have it.
Under 16 ft
One wearable PFD (Type I, II, III, or V) per person; an efficient sound-producing device; navigation lights when operating between sunset and sunrise; and the engine cut-off switch link used when on plane, on a boat that has one installed (engines capable of 115 lb of static thrust, roughly 3 HP). A fire extinguisher is required at this size too, unless the boat is an open outboard whose construction cannot trap fuel vapors; a gasoline inboard, an enclosed compartment, or a permanently installed tank all trigger one. Gasoline engines add a backfire flame arrestor (inboard or stern-drive) and ventilation of any closed engine or fuel-tank compartment. Night-use visual distress signals only. No Type IV throwable is required below 16 ft.
16 to under 26 ft
All of the above, plus one Type IV throwable device immediately at hand; one 5-B (or 10-B) fire extinguisher, or a still-in-date B-I on a 2017-or-older boat, though an outboard boat of open construction with portable fuel tanks and no vapor-trapping compartments is exempt from the extinguisher; and day and night visual distress signals, which can be a non-pyrotechnic orange flag for day plus an approved electronic SOS distress light, or three day and three night flares. The engine cut-off switch law applies to this class.
26 to under 40 ft
Wearable PFDs plus a throwable; either one 20-B, or two 5-B (or 10-B) fire extinguishers; the same day and night distress signals; an efficient sound device, with a four-second-blast horn audible half a mile required only from 39.4 ft / 12 m up; navigation lights for night and reduced visibility, where a power-driven boat of 12 m (39.4 ft) and over shows separate red and green sidelights, a white masthead light at least 1 m above them, and a white stern light, while a power boat under 12 m may instead show an all-round white light with sidelights, plus an all-round white anchor light at anchor; ventilation and a backfire arrestor on gasoline boats. A MARPOL oil-pollution placard and a MARPOL garbage placard are both required at 26 ft and over, whether or not there is a galley. A Type I, II, or III marine sanitation device is required if a head is installed, and in a coastal No-Discharge Zone only a holding tank (Type III) may be used.
40 to 65 ft
Wearable PFDs plus a throwable; either one 20-B and one 5-B, or three 5-B (or 10-B) extinguishers; the same distress signals, sound device, and placards; navigation lights with greater visibility range; a current Navigation Rules reference carried aboard, required from 39.4 ft / 12 m up, where a paper copy always counts and an electronic copy only on a system that meets Coast Guard standards; an oceangoing boat 40 ft and over with both a galley and berthing also needs a written waste-management plan; and the same head, ventilation, and No-Discharge-Zone rules. A bell is added to the whistle only at 65.6 ft (20 m) and above, so no bell is required below that.
Disposable (non-rechargeable) fire extinguishers must be removed from service twelve years after the manufacture date stamped on the bottle, and checked yearly: gauge in the green, pin in place, nozzle clear, no corrosion. Rechargeable units have no fixed expiry but need regular professional service. A 10-B is interchangeable with a 5-B, and one 20-B replaces two 5-B units. Pyrotechnic flares carry a 42-month service life and expire on the date stamped on the casing, which is the single most common reason a boat fails a Vessel Safety Check, so keep them in date. Federal law requires children under 13 to wear a PFD underway unless they are below deck or in an enclosed cabin; where a state sets its own child-PFD rule, that rule governs. State rules can exceed these federal minimums, so confirm with your state boating authority, and book a free Vessel Safety Check from the Coast Guard Auxiliary.