The sea stone ships make it possible for the marines to enter the calm belt without getting swarmed by thousands of sea kings.
Traditionally, Amazon Lilly was safe from invaders (or nosy men) because of its position right smack in the middle of the calm belt.
But now the new sea stone technology would allow the WG to attack the island, and thus Hancock's title helps it avoid that fate.
seas stone coated ships will help the marines to take shortcut to the newworld without going thru the first half of the grand line