I prefer to lay back with the back of my head against the direction of oncoming waves. It seems to me that trying to stay vertical would be more uncomfortable.
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One of the more illustrative vids on the harness:
I wear a rubber belt under my backplate. I prefer it to the integrated weight pockets, which make the waist belt hang down when you put the BPW on. Only downside to belt is remembering to put it on ... when i am rushed sometimes i forgetHave a think about how you arrange your weights. With a crotch strap a weight belt is awkward if worn under a one piece harness. If using pouches not so much. Removing a weight belt prior to pulling yourself back into a rib requires a bit of care.
Make sure buddies know how to get you out of the harness if needed. Carry a line cutter that'll work on webbing and show them where it is.
Enjoy diving a wing - underwater trim is so easy and you don't have all that BCD bulk in front.
I was never able to do that in a jacket either. It always slipped up (jacket shoulders around my ears, my armpits against the pockets) no matter how tight the waist strap/cummerbund was fastened. (This was with a weight belt, rather than integrated weights.)