I don't think you can blame inch-pounds for this one- even though we rationally know it is heavier than most materials we don't "think of" water as heavy because modern society gives us the luxury of seldom having to handle more than a thirty minute or so supply of it at once.
then again, anyone with a garden or balcony that has to be watered with a watering can can [sic] attest to how heavy a little bit of water is. If you have a small 100m² garden and it rains a modest 1mm (per m² that is) that's 1L/m² equivalent to 1kg/m² or 100kg total mass of water. When it has rained 1mm people call it "three drops of rain" and they will have to water their plants anyway. That's already 10 cans @ 10L each to lug around.
The other kind of people who know about the weight of waters are people with campers / trailers and people with fish tanks in their apartments. The maximum allowable size for a fish tank in the middle of the room is not that much.
I wasn't actually commenting on the "heavy" part but the "8 pounds per gallon" part. I can do these conversions on demand but, besides never being the native way I think personally, there are simply more numeric conversions you have to do when you use this system whether or not it's native to you.
