Assuming the containers you have in mind are prisms with their horizontal base being a hexagon or pentagon, with vertical flat sides, the volume will just be the area of the base times the height (depth of water).
You haven't told us what dimensions you know about either container. If you happen to know the base area, then I've just told you all you need to know. If you know the volume when full, then the volume at a certain depth will be proportional to the depth. Otherwise, you'll have to tell us the details. In particular, are these regular polygons (all sides and angles equal), or not?
And if the base is not horizontal, we'll need lots of information!