Marine environments are tough. West Marine is not inexpensive.
That said, we looooooooves our shock cord, and might to better off with less of it following analysis of just how much or little is required and what strength it should be. Foks who are concerned with breaking strengths of regular cord freely include shock cord of unknown but almost certainly lower initial strength and unknown durability compared to what they would be comfortable with if they knew.
I'm glad to hear of specific recommendations for sources.