Both of them have the "島" kanji (but pronounced differently) at the end. Fishman Island is Gyojin-toukontoi (魚人島), while Onigashima is (鬼ヶ島). That ending is quite common in One Piece, but for these two the nomeclature is "race of creatures" + 島. I wonder if Onigashima is similar the fishmen (or minks), where there are different races/mixtures of Oni, like Fishmen Oni, Sea Dragon Oni, etc...and they can all intermix. If that's the case then Jack would be a race of Oni, rather than a half fishman. We also know that Oars died in the "Land of Ice" because he didn't have proper clothing, while Kaido is properly dressed. I wonder if the Oni have been occupying this territory ever since Oars died and landed here with his bands of people. Stephen from VIZ actually pointed out that Kaido's title "Hyakuju no Kaido" (Hundred Beast Kaido) is similar to the Japanese translation for "King of the Jungle" nomer for the Lion, 'Hyaku-juu no Ou", implying Kaido is a bit like the King of all the Beasts in Onigashima.