I use 7 - 55 gallon barrels. Any type of container will help. But it takes a lot of solar heat storage to do any good.
Someone here calculated once how much water it takes to produce a BTU of heat. How many buckets do you plan to use and what is the size of your greenhouse?.
I think Weebus can help you out on this. He's pretty much science savy.
Have you read the thread about using a solar pool cover to help hold heat? See 'My page'.

The bulk of water in there will help, but I have found you need to artificially move the heat around.

The MAIN benefit of having a water mass in a greenhouse where you are is keeping the place at the 33-31F mark as in just about freezing.

Just warm enough to avoid a killing frost.

For cool climates (4 month winters) Shane Smith recommends that you have 2.5 gals per sq ft glazing area in an attached GH and 3 gals/sq ft glazing area in a freestanding GH.

Thermal storage in BTU/sq.ft./Degrees F:
Brick - 24
Concrete - 35
Earth - 20
Stone - 35
Wood - 10
Water - 63

Thus, water can store twice as much heat as other materials.

If you put them anywhere but the south wall, some people recommend painting the side of the container facing into the GH white to reflect the light in, and the side facing the glazing black to collect the heat.

SB