Zunesha is many miles and miles away from Onigashima though, just look at the first page of the chapter. And I doubt Zunesha can walk on land considering his legs. It just happens that Zunesha's hearing is that good, LOL.
And I think it's a good moment to start revealing these big pieces of the puzzle.
I mean the size of Zunesha likely means he can spray water quite far since his bathing himself was going up against gravity, where it would presumably be more horizontal against Onigashima. or his size means he can probably cover ground quite quickly, if requested by Momo or Luffy.
Also Zunesha will be able to do anything in the story pretty soon. Animals in OP have always seemed so lively and colourful. Pre Zou arc if Oda said he was doing an arc on a giant elephant I don't think anyone envisioned it to be so decrepit, sunken eyes and frail looking and depressed.
But the reason for this I'm fairly certain now, since his reveal he's joyboy's partner in crime is that Luffy will use his awakened powers and make Zunesha a lively animated elephant