The body doesn't really have any way to retain [liquid] water without causing medical issues. There's no storage compartment except for fat that could be metabolized into water.
You can also store water, for a short time, mixed into fiber in food that was recently eaten.
I'm not implying I have 'water pouches' or anything as silly.
1.5L of water in my stomach gets processed as quickly as my kidneys can manage. Thus I urinate out a large % of that liquid intake before use can be made of it.
Continually drinking small amounts doesn't overwhelm my stomach and therefore it feels like I urinate less, and 'retain' that water.
You can also store water, for a short time, mixed into fiber in food that was recently eaten.