A good time for a safety reminder David.
Having been in a couple of times once in the Thames Estuary in my late teens I would not be on the water without one, the Thames incident was caused by getting too close to the sandbanks we were fishing to and getting pitched out of a small boat by a wave I learnt my lesson. Fortunately it was just a case of getting to the bank and then running the boat up to pick me up. The second was on a Topper Dingy where I was trying to teach a novice while looking back to try to give instructions we had a crash gybe and the boom hit me across the bridge of the nose and knocked me in. Fortunately this was in warm water not too far from shore and the shock of the water brought me round. One reason I have an aversion to two man sailing dingy's.
While the Broads may seem benign the biggest issue is getting back onto your ship but a Life Jacket buys you time and could well save your life if you have had a bang on the head and are wearing the right type.
I would always recommend David's number 4 the 150N version can be bought quite competitively, if you are baulking at the price, ask yourself what price your or a family members life!
You can buy a version suitable for the Broads at sub £40 e.g. http://marinestore.co.uk/Merchant2/merc ... _Code=mrst
For our offshore work we have invested in a 275N version with Sprayhood