Getting a six year old to put on or keep on a bulky life jacket can be a battle if you allow that child to think no life jacket on the dock or boat is ever an option, just like seatbelts. With the recent death of a young girl at a nearby marina should be the wake up call to not be lenient.
I heard it time and time again, "Auntie I don't like it. I'm hot. Why. I know how to swim." I start with the authoritarian approach that you wear it, no option or negotiation. Later, sometimes way later, once they understand that I mean business and will not weaken. I explain to them why, and I will now use this detailed story because stories have the most impact for memory.