HydroCore or Aqua Bag...
 

... Have you tried it? Do you like it?

Dimok Aqua Bag
Tidal Tank Aqua Bag

HydroCore
HydroCore Flows
HydroCore Revisited

Ok, I checked the price for the HydroCore, and I think it is expensive, I think it is $109. The Aqua bags are priced better, I found this one that come with two sizes, up to 12 lbs and 49 lbs Aqua Gym.

And this one Yes4All Aqua Bag.

Tidal Tank -The Original Aqua Bag on Amazon

I have a sandbag and I love it, so I am tempted by this. And, I could use my sandbag DVDs.