Luke, you are quite correct, due to the different denities of air and water.
If you try to lift a 6lb weight using a 6lb line on dry land, you should be just about OK, depending on the quality of the line.
However, if you put a12lb weight on the same line and place it in a bucket of water, then the 6lb line should still lift it,
as the density of the water will help support the weight.
Another way of thinking about it.... if you dangle a 2 oz weight from your rod and have it suspended in the water,
Look at the bend in your rod tip. Now reel-in until the weight is clear of the water; the bend in the rod tip should be greater,
as the water is not supporting the weight.
That's just the straight physics with no rods or reels envolved.
Once you add balanced tackle, shock absorbing properties of you rod tip, reel clutches etc, it swings even further in the anglers favour.