You can (slam) dunk a basketball, but you can't dip it (to my knowledge).
You can go for a dip in the ocean, but not a dunk.
Taking your question on its face value, 'dip' is a more common word than 'dunk', since it has more uses.
If inserting a piece of food into liquid is your context, I'd guess maybe 'dunk', at least in Aus.