Turn the question around!

A 1 litre plastic container, full of water, will be just about neutrally bouyant. If you add any bouyancy to it, it'll float. So...

If you're aiming for 20-25kg of water ballast in the barge, then you'll need a barge of "just larger than" 20-25kg displacement in order to float. The more bouyancy, the more it'll float - and the better the freeboard.

Think about a basic box. Let's assume the surface area of the box base is 80*25 cm. That means for every 1cm of depth in the box, you can hold ~2000cc of ballast, which, if water, will be 2kg.

To just float 25 kg will need 12.5cm of hull in the water. Add maybe 5cm of freeboard, and you get 10kg of positive bouyancy.

Andy