It's easy to figure out how much something weighs when it's in the water if you can estimate how big it is. Let's say you see a log floating in the water, you estimate it to be 50 feet long, and about 2 feet in diameter. The volume of the log is 50 feet * Pi * Radius * Radius (you know, Pie Are Square). Radius is half the diameter so, 3.14 * 1 squared is 3.14. Multiply by the length, so 3.14 * 50 = 157 cubic feet. Now if the log was just barely floating, that is, no portion of it was sticking out of the water, the rest would be easy.

A white guy named Archimedes came up with a law about 2500 years ago that said that an object placed in water displaces it's own weight in water. So, as long as the log is floating, and not sitting on the bottom then all we have to do is know how much water weighs. That's 62.4 pounds per cubic foot. So, 157 cubic feet times 62.4 pounds is 9796.8 pounds or about 5 tons.

Now, if the log is floating high in the water, you have to figure out how much of the log is out of the water, so for example if the above log was only 50% submerged, then it would only weigh half as much, that's why it is floating above the water, it doesn't weigh as much, or 2.5 tons. If only 10% of the log is submerged, then it would only weigh 1000 pounds, half a ton.

This is the reason that wood floats, (except Natalie Wood), it's less dense than water, so it floats on top of the water. If a log stays in the water too long, it becomes "water logged" and sinks. Once that happens you can't estimate how much it weighs, all you know is that it weighs more than water, but unless you put on a scuba suit and place a scale under it on the bottom, you can't know how much it actually weighs

Archimedes figured this out back in ancient Greece 2500 years ago when the rest of the humans on earth considered the problem of wood floating on water too difficult to comprehend.