USCG guidance emphasizes an approved device of the correct size, appropriate for the activity and in serviceable condition. A wearable device must be accessible rather than buried under equipment.
Try the fit, read the label and understand the limitations of inflatable or specialized designs. Children and activities such as personal-watercraft use may have additional requirements.
Wearing the device while underway removes the delay of finding and putting it on during a sudden capsize or fall overboard.