Mr. Narked might like to comment here......
I was taught, when doing some diving training, in metric, (originally learnt in Imperial, but metric is soooo much easier) that water weighs exactly 1kg per litre.
Now without checking anything, petrol weighs less, but if you include the container....
What I am really getting at is that, if for instance, you have need to carry two 20 petrol cans from the garage to the slipway, near as b****r it, it will be nearly 40kgs. or for the less technical amongst you- bloody heavy!