Graves are the remains of hedgehogs which has died. They are created when a hedgehog dies on land because its health got down to 0. There are no graves for hedgehogs which have drowned.
Graves as such are not very interesting, they just stand around and fall straight down when land is removed below them.
But graves can be used by a resurrector to bring the owner of the grave back to life.
Each team can have their own grave style. The HWKB has an article on creating your own custom graves.