I am currently transcribing the "General instructions for Commanding Officers of Naval Armed Guards on Merchant Ships". Here is an excerpt from that volume on the subject:
5604. Men should be instructed to leave the collar or top straps on their life jackets untied, and to hold their arms down close to the body, and to jump into the water feet first. If the collar or top straps are tied the upward force of hitting the water forces the jacket up, and the straps under the chin are pushed upward with enough force to render a man unconscious. By holding their arms down close to the body the force will be taken under the arms. A preferable method is, of course, to ease into the water by line or ladder at a point nearest the lifeboat or raft.
5605. Steel helmets must be removed when jumping overboard. If the chin strap is secured, the upward force on contact with the water is sufficient to break the neck or cause serious injury to personnel.
Hopefully this volume will be completed in the next few weeks and posted on the hyperwar web site along with the other volumes I have already posted. Interesting reading!