In case anyone is wondering about Hydro's fate.
Well in the raws for the next chapter 138, the flashback concludes with him putting a valiant last stand against their pursuers. Meanwhile Kagura gets safely washed to shore.
We find out Hydro must have got captured cuz after the flashback ends during chapter 138, those Biove scientists lead Kagura, Reve and Diva into a room with test tubes.
In one of those tubes is Hydro. Hydro is in really bad shape since he is missing all his body parts below his waist and both of his hands as well.
It's reasonable to assume the people of Biove have been lost likely been experimenting and torturing Hydro given his current condition imo.