First we know Jack is alive somehow in the ocean. Many people believe he’s a fishman or part fish man, other’s argue he’s simple holding his breath. For the purpose of this theory it doesn’t matter, he’s alive and in the water, that is the key.
Second we know Zunisha twice a day such up ocean water into his trunk and rains it down on Zou, and in fact that water carries a lot of fish and even sharks with it. I feel Oda included this for a reason and that is what I think could happen next.
I believe that Zunisha will inadvertently such Jack into it’s trunk and rain him down onto Zou, but in doing so Jack will injure Zunisha trunk to try and achieve his plan to injure or incapacitate the giant elephant and in doing so will in fact bring a drought to Zou by cutting off it’s source of water, there by literally living up to his nickname. While he may not be able to move in the water, there will be air mixed in at well and if he get’s into an air bubble he’ll be able to attack Zunisha from the inside out while riding the stream to Zou.
The biggest problem with this theory however is location. Jack is presumably a distance away from Zou at this point, with the old elephant still moving, albeit at a slower rate.
This could however be addressed a number of ways.
1. Zunisha isn’t moving in a straight line, but rather wonder’s around a certain area for a certain amount of time before moving onto a new location, Elephants do this in real life in certain conditions, they move around a specific area for part of the year, and revisit some of the same locations before moving on to another general area where they repeat the process.
2. Jack may not be able to move on his own, but ocean currents could easily move him around in the water, and Zunisha probably isn’t that far from his current location at any rate, it was injured fairly badly.
So yeah that’s one of the way’s I think Jack could get back to Zou. Not great but I think the idea has interesting potential.
*Theory by Herr Krokodil