The old Navy method was to
Drop feet first toes pointed
Cross legs at the ankle
Cross arms
Grasp opposite shoulders
Tuck chin into chest
Keep elbows tight
DO NOT repeat DO NOT wear life jacket
Grasp a strap to the jacket

You might be able to wear an inflatable style life jacket

Crossing the ankles keeps the impact from spreading the legs and breaking them. This minimizes the the impact on sensitive areas. It also reduces the risk of hydraulic impingement to lower body openings. Yes the entry can cause internal organs.

You want to be as streamlined a package as possible. You want to pierce the water as cleanly as you can. Not land splat. Cannonball dives don't work. At any significant height it is like hitting concrete.

You hold onto the life jacket. Yes when you hit the water you will lose it. But it should be near when you resurface. The jacket is bulky and meant to float. It will stay at the surface. If you are wearing it even or especially with crotch straps it will greatly magnify the impact. It will try to keep you at the surface and remove your head in the process along with other body parts. There will be no give such as a chute opening in skydiving.

Although things may have changed in 30 years since I learned how.