Since Dressrosa it was a mystery how only Luffy was able to hear Law's voice but thinking about Luffy's talking to Momo while he in the sea it seems Law might have the same ability and used it to let Luffy hear him.
even the bubbles are the same as Momo heard Luffy.
and there is a panel as they were in Zoo while going to the whale tree , he has thought bubbles , as he might have realized that Momo has the same power
and there is a panel as they were in Zoo while going to the whale tree , he has thought bubbles , as he might have realized that Momo has the same power