About the mysterious sand bags

I'm very interested in these sand bags. One of the guys in the studio is a retired rigger so I asked him did he know what they are used for.  He thought they might be used as ballasts (as weights on a boat, so that weight is distributed more evenly and it floats). Or to block leaks. He wasn't totally sure, he has a friend who works in the Dockyard so I'm going to go down there on Monday morning to see if I can find out more.