These words are pronounced the same but have no relation to each other.
To scull, as a verb, describes the action of rowing a small boat, especially a certain kind of long, narrow row-boat called a scull, using oars also called sculls. Sculling can describe the action of rowing any small boat, or the sport of racing these long, narrow boats, which is a traditional competition in some universities, especially in England.
A skull (noun) is the part of your skeleton, made of bones, in your head—the part that protects the brain, plus the jaw-bone—in any animal, not only humans. It is a simple, straightforward word, except that sometimes we use skull to mean mind:
I could not get these ideas into my skull.
It also appears in the terms skullduggery (sneaky, dishonest behavior) and the skull and crossbones, a universal symbol of danger—and pirates!