A typical PPL UK water crossing would be the English Channel. The shortest crossing is Dover - Calais 20NM, but both Eastbourne - Le Touquet and IOW - Cherbourg approx 50NM are examples of other popular crossings. Most UK pilots I know of do those crossings with a life west only.
Before I was allowed to cross the Channel I had to do a "Channel Checkout", a requirement on our group aircraft and most club aircrafts I know about. In our group it forms part of the insurence requirements. As already has been mentioned; you might find yourself in imc like conditions on a clear day.
If you are only flying over water for a couple of channel crossings every summer I wouldn't worry about investing in anything else than a life west and a PLB. I'm a firm believer in mathematical statistics
