I have over 1200 scuba dives and play underwater hockey (you hold your breath and push a lead puck on the bottom of a pool). I have been flying float planes off and on for over 30 years. A few years ago I got offered a free spot on a eggress course set up for light aircraft.
They have a mock up cockpit in which you sit strapped in. The instructors then submerge the device tumble it around and leave you upside down. You are told to get out when it stops moving and take the life jacket in the seat pocket with you.
Going in I thought how hard could this be ?
I have thousands of hours under water most of it holding my breath.
Well the first time I never got out. The instructors finally had to right the device and bring it to the surface. After six tries I could get out no matter what they did to me.
The two take aways
1) Before taking the course if I had wound up upside down in a submerged cockpit I probably would have died
2) I now always wear a constant wear type life jacket anytime I am near water in a single engine airplane
Bottom line: If you are flying over water not within gliding distance of land or flying in a seaplane take an egress course.