That’s a cold lesson to learn, I guess you will never leave your back zipper open again
I don’t think you will have the same problem if you put a small rip in your sleeve or leg (tough to do with these drysuits) because the suit would not fill up as fast or as much as leaving the full zipper open (zipper length is about 2 feet long and it is a huge space to leave accidently open to the water). If I felt any cold water in my drysuit while on the water, I would head straight back to shore without hesitation.
I would say the drysuit is neutrally buoyant, but there would be a tipping point as more water enters the suit where it would turn the suit negitive buoyant. If in doubt put on a life-jacket. In cold waters/weather, I always wear a (kayaking) life-jacket, because doing any sport in cold water has its risks…drowing due to the cold water would make for a bad session.
Zipper breaks are also pretty uncommon. The O.R. drysuit zippers are similar to diving drysuit zippers and as long as they are taken care of, they will last. I do a lot of cold water diving with O.R. sister’s company (Frank Whites) drysuits and they have always kept me dry down to a depth of 200ft and as cold as -1C waters. In fact, I think they use the same brass zippers so I trust them completely.
The drysuit is like any piece of kite equipment, check it out(inspect it) before you hit the water. If if looks old or grubby, replace the seal or patch the piece.
Whoa, that turned out to be a longer post then I intended.