Davy Jones fruit lets Xebec control the ocean making him the biggest opponent of all devil fruit users
Is it the Pirates of the Caribbean incarnation, where he can submerge and control the waters (i.e he can make whirlpools, like a certain flame scarred individual)?
Is it the Davy Jones locker fruit, which the Davy Jones locker is a metaphor for an endless abyss (i.e. bottomless pocket dimension fruit)?
Is it a PART OF THE SHIP PART OF THE CREW fruit that lets him basically mindcontrol people?
Tons of possibilities there.