My understanding is that the EPA does not allow you to dump human corpses in lakes. You also need to go >3 miles out to sea. https://www.epa.gov/ocean-dumping/burial-sea

Some entity was also probably not okay with composting bodies in your backyard —till this.

In California you can’t bury a corpse in your backyard either. But in most states you can.

I wonder how big of a trebuchet you’d need to launch a body 3 miles.

I don't even know if that's possible. 3 miles is a long way, and bodies are squishy and... easy to separate, relative to a rock or bullet.

The speed you would need to get that far might be high enough for the force to tear off limbs or to shred the skin. The chances of you getting a permit after you put "might rain human limbs or remains on the beach" is very low.

You'd probably have better luck with little solar-powered, GPS-guided dinghys or kayaks that can go out 3 miles, drop the body and come back.

Lake Superior is perpetually cold and near-sterile when you get deep enough. Not the greatest place if you want bodies to decompose.