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back zips are for surfing (because you lay on the board much of the time) & front zips are for scuba, especially in warm water or areas with thermoclines (because if you get too warm, you can unzip & cool down)
Surfering are mostly a surface sport. Mostly with the torso out of the water except for the little bit they fall into the water. Paddling out, hanging out waiting for the right wave, sitting on the board with feet in the water. Hands if paddling. Actually riding a wave, just some splash.