Interestingly enough, Hawaii actually has its own subspecies of cobra known as the Yellow-Bellied Sea Snake. While it's by no means similar to the cobras we are used to, it's quite possible that Arbok would take on a Water/Poison typing to adjust to the abundance of water that Alola has.
One reason for this could be that Arbok was simply incapable of relying solely on land-based prey to satisfy its appetite. It could also explain why Raichu might take on its Electric/Psychic typing as it would now be able to counter the evolutionary change of Arbok.
Food - or in this case - prey for thought...
(Snake photo retrieved via Wikipedia)