Depends on the laws where you are. Maritime NSW doesn't consider us a vessel that requires it per the link below.
maritimemanagement.transport.nsw.gov.au/lifejackets/index.htmlOf course, that doesn't mean event insurers won't insist on an organiser putting it in their race rules to get cover to minimise risk. So it is basically whatever the organiser will accept in order to let you participate.
Real vessels, (we must be figments of their imagination), need to have one on board when they go more than 100m off shore even in the harbour, and wear them in some circumstances such as crossing bars.
Mind you it makes sense to use your noggin' and wear one when risks are higher e.g. when not in a big group, in high winds, well off shore etc. If it's that windy and cold it might just keep you from getting cold or hypothermia too. From my experience, sh1t does happen now and then. Besides, on a slow day Maritime patrols sometimes pull you up to have a chat with you but won't bother if they can see you've got one on.
I've got an Ultra inflatable that goes over the shoulders myself. Folds up quite small, drys easily and can be taped to the deck if you don't need it on you.
