First, stay away from that lady at the dive shop- she's clueless beyond measure.:head
Opencell refers tot he lining on the inside of the suit. Most scuba suits, besides something like a Henderson Gold core, will have a nylon lining on the inside of the suit fro strength- but this allows water to flush through the suit. Opencell is bare rubber and it sticks to your skin like glue- so NO water can flush through. Some open cell's can be sprayed with different coatings to make them easier to put on, but all of these will wear off after a couple of hindered dives. They cost more, and reduce warmth, but may be a good thing for a newish diver because they make the dressing process easier. Opencell, even in spray coated, is so sticky that the only, decent, way to put it on is by spraying the inside of the suit with something like hair conditioner, and water, to aid in sliding it on- this is called lubing the suit.
Once the suit is put on I have been able to stay completely dry down to 60'. Below 60' the suit itself compresses and allows some water to flow in, and out.
In warmer water I wear a 3mm- as in anything above 58F. If i only have my 5mm with me and the water is warm, and this did happen to me once this spring, I just flush my hood every 20 minutes or so. I never take it all the way off, even though I could, to keep the seaweed out of my hair and ears. rofl Flushing the suit every 20 minutes, or so, seems to really do the trick.
I was diving up in Vlias county the end of July and hit some weird water temps/thermoclines. It was too warm for my full 3mm, but too cold to just wear a vest. I ended up just wearing my 3mm jacket over a pair of board shorts and was very comfortable. I was cool enough with my legs exposed, but didn't freeze out when I dove below the thermocline- which was an easy 30 degree temperature drop. that's another advantage to a two piece suit over a one piece- you can layer for the occasion.
Jon