Identify the Ingot

My son pulled this out of the river with a magnet. I haven't weighed it, but I would guess it's about 50lbs. It's got a roughly trapezoidal shape, and the bottom is 18.5 by 4.75 inches.

It has what looks to be an H stamped in the side, here.

No idea how many years it's spent in the river, but this is what it looks like after it's been power washed.

Does anyone know what it is? What kind of metal, and what is it used for? Why does it exist? I have some guesses, but I'm wondering if anyone just knows what it is.