Paddleboarding in the US:
find your water

• When a paddle board is a vessel. Under federal law a stand-up paddle board counts as a vessel only when it is used beyond the narrow limits of a swimming, surfing or bathing area. Inside those limits the federal carriage rules below do not apply.
• Life jacket on board. Once it is a vessel, 33 CFR 175.15 requires at least one wearable U.S. Coast Guard-approved life jacket (PFD) on board for each person, and 33 CFR 175.19 requires each one to be readily accessible.
• Wearing it. The federal rule does not require adults to wear the life jacket; it requires it on board. Children under 13 must wear an appropriate Coast Guard-approved PFD while the vessel is under way, unless they are below decks or in an enclosed cabin, which does not describe a paddle board.
• Whistle and light. Coast Guard guidance also expects a sound-signaling device such as a whistle (the human voice does not count) and, after sunset, a portable white light or flashlight.
• States add their own rules. Many states set stricter requirements, such as mandatory wear, specific PFD types or age rules, and they differ from state to state, so there is no single US rule on wearing a life jacket. Look up your state's boating agency before you paddle.
• In an emergency. Call 911. The U.S. Coast Guard monitors VHF channel 16 if you carry a marine radio.

Sources: 33 CFR 175.15 and 33 CFR 175.19.