Things float on water if they are less dense than water. The density of lead is usually about 11 times that of water. So no, it can't float as a pure lump of lead in a pure tank of water. However water becomes denser if you fill it with salt, while lead could have a lower effective density if you managed to spread it out into a large enough sheet. So maybe in very salty water over a large surface area, you could just about make lead float.
