I did a marine research project in college on Tectarius muricatus which is a beaded periwinkle. From what I saw, some periwinkles actually live on the rocky seashore part of the time. The particular snail I studied stayed in the seaspray but didn't actually go into the water. Perhaps, some types of periwinkles are just not suited to living in the water 24/7. That might explain why so many people see periwinkles climbing out of their tanks.