As Steven suspects, the "water" isn't really wet at all; it doesn't seem to have any discernible texture. Still, it holds them up as though they're both in water, bobbing the tubes softly and allowing them to glide alone the surface.

As for where he got the pool... Tamaki seems to ponder this for a moment. Up until now, the pool simply was. But now Steven has put a reason to wonder where the pool came from into Tamaki's head, so the dream provides one.

"It was a gift." He lazily swishes the stars below, watching them glimmer in response. "Because I wanted to watch the stars with everyone."

Normally one wouldn't go stargazing quite like this, but dream logic is formidable.