Natasha didn't particularly dislike her larger, water-filled ass. It was possible that once she was aware of it for longer, she'd grow to dislike it, but for the time being she was at least intrigued by the sensations that it presented her--and, more importantly, why she was absorbing water and storing it in ways fat would normally be stored.
The fact that it made her ass big and jiggly (or perhaps sloshy), as though it were full of fat, without it actually being fat, made Natasha excited in ways she didn't fully understand. Maybe it was the few years of being harassed by other girls for being fat while still remaining skinny--the idea of having such a big lower body and all of the sensation (give or take) that accompanied it without having the massive calorie surplus needed to create such a lower body was something she couldn't pass up.
That, combined with her need to test a sudden concern--namely, whether or not she would absorb water through her skin the same way--led Natasha to pour the slightest bit of water on her aquatically padded rear end. She pulled the sweatpants down a bit, struggling to get them over the newly-formed shelf of her ass, and tipped the bottle by the tiniest increment she could manage. It was still more than she intended, but her slight error in measurement served only to excite Natasha more--slowly but surely she was beginning to like the idea of swelling up with more and more water. As the stream of water from the bottle landed on her full cheeks, it simply disappeared. Mere moments after, Natasha felt and saw her water-filled buns surge outward just a bit more--not as much as she had grown before but it was definitely noticeable. She was developing quite the booty, as the now-quite-snug sweatpants seemed to attest.
Two things were now obvious: water could be absorbed through her skin as well as by drinking it, and Natasha wanted to be bigger. As she walked out of the house, though, a few more things came to light.
For starters, there was the fact that her pants were now way too tight. They fit, but they were pretty much a second skin after her most recent growth and wouldn't last much longer if she grew any more. Plus they were incredibly uncomfortable, especially as they started to wedge into her deepened crack, which was only aided by the constant jostling motion of her cheeks. Water moved a lot more than fat did, or at least as much as fat seemed to, and the sloshing, quivering, bouncing motion of Natasha's ass was having a clear toll on her walk. She was having to sway more to simply move, and the swaying only increased the motion of the ocean behind her. She enjoyed the feeling, but hated how uncomfortable it made her pants, and how neither she nor the rest of the world could enjoy the spectacle of her enhanced assets. She wanted to wear something that would restrain her ass a bit less, both for comfort and for the visual she would no doubt create.
The question is, does she go to find new clothes first, or does she seek out the old man?