On a new submersible course that I am installing this week, I was planning on using 5/16" shock cord for the top 4' connected to skier and gate markers. When the course is submerged this particular shock cord is stretched to 160%. I have adjustable shock-cord hooks for making large adjustments, but the day-to-day 3" water depth changes would be evened out by the shock cord. If it lasted 5 months that would be good enough for me. My concern is that it might forget it relaxed length when its stretched to 160% almost all the time.