I have a rock bag like that but have not used it yet. I have dog tie out screws in each ski that I use all the time when we are sitting in shallow water. Get them cheap from Home Depot. I think Harbor Freight has them even cheaper. Works great! http://www.chewy.com/dog/aspen-pet-...at. I think I'm gonna order a couple as well.
I've been curious about this. Do you put rocks in it, or just fill it with sand? I will be mostly only be using one in shallow water (sandbars) and was trying to decide between these and those sand spikes that you drive into the ground and tie up to.