Water-Sand Dualization

So allow me to talk to you about how some water people live inside castles.

The Middle Child and the Raining King

We start in the grand castle of San Castelo, Castelo de Areia, a castle made of sand where the royal family sits. I say it's a castle made of sand, it is the castle made of sand and the only castle, let alone structure, in San Castelo. Every building in San Castelo must be made of sand, but builds under that rule crumble within a week.
Around this time, the family's middle child was talking to a friend, a corner of the castle's kitchen. They tell them their perils and the castle corner asked them to talk to the Raining King. With what might happen to his castle, he obliged and travelled to the Land of Rain, where the king was waiting for him. The Raining King tells the middle child to head home right away, where he first complained about the trek here, but then obliged and head back.
At the Land of Sandcastles, the middle child was surprised. There were more structures than ever before, and houses were being built again. The child asked the corner what happened when he was gone, and the corner said that their friends just dropped in to help.
And that is why the water helps strengthen sand.

Utility

The beings of water go into anything sand to strengthen it. It's discovered as a way to strengthen buildings in San Castelo, however, some are using it for their bodies to endure many hits, willingly or not.

Access & Availability

It's practically simple, but hard to stay put. Water people are free spirits and tend to wander around the Black Marble as they please, longing for adventure. That is why after a few days, the structures in San Castillo would grow weak and fall.

Complexity

It's kind of complex. It depends on how much water to go into sand that won't turn either of them into a wet mess. Too little water wouldn't make a difference, but too much can topple the structure. For every furnished room in San Castillo, there has to be one person made of water there to strengthen it.