I put my life jacket on when I leave the car and take it off when I get back to the car.
I have a good auto inflatable mustang with integrated harness but usually just wear a standard type two near coastal.
I am in the extreme minority as far as I can tell.
The fact that I'm not a good swimmer is only a small factor. I doubt if many people can swim effectively fully clothed for more than a few minutes. I doubt if even ten percent of boaters could even tread water in a pool for an hour.
I sail in long island sound. The water temperature is usually too cool for someone to last very long.
If I end up in the water it is probably the end result of more than one thing going wrong. The amount of energy someone expends in just a minute or two during an event would also probably affect their ability to effectively swim.
Swimming in a pool and in open water is not even remotely the same.
Falling off a boat often entails some injury.
I am almost always alone or the only boat handler on board. If I go over I may be on my own for a long time.
Coast guard personnel wear life jackets and they are smarter than me.
ASA schools require everyone to wear life jackets, they are smarter than me.
I want to sent a good example.
The most important reason however is Soundings Magazine.
Soundings is a Long Island Sound boating magazine. At least twice a year for as long as I have been getting it they print the same story with names changed.
It goes something like this:
Carl was 59 years old and a life long boater. He was a member of the fire department until he retired after 20 years service. He was devoted to he boat Summer Sun which he sailed for the last 40 years. Yesterday his boat was found washed up on the noname beach. His body has not been recovered. Carl was not wearing a life vest.
This story runs almost every summer month every year.
Off-shore I wear the mustang and use the harness at night or in gusty conditions.