A cubic kilometre of untainted seawater (pacific island) contains about 3.5g of Pb. A battery contains about 9000g of Pb. So one battery could affect a lot of seawater IF it were to be soluble.

  Lead concentrations in offshore waters are ranging from 0.0035 (American Samoa, South Pacific Ocean) to 0.150 pg/L. (Western Mediterranean Sea; Copin-Montegut et al. 1986).

  A lead-acid car battery typically contains 16-21 pounds of lead

  For comparison: average concentration of 20 mg/kg of soil

Very important habitat restoration for electric eels, actually.