After Taiga sees Ami trying to mess with Ryuuji, I think she does get jealous. Taiga might not realize it herself, but her relationship towards Ryuuji changes quite a bit. First of all, there is the lack of physical violence and verbal abuse compared to previous episodes. The fact that Taiga is bothered enough by the prospects of Ryuuji and Ami messing around puts her into a depression (one that Yasuko can notice) later on at dinner time show's that has been affected. Furthermore, how often does Taiga yell at Ryuuji to shut him up? Or just physically hit him to stop talking? Now compare that behaviour to: "My, why are you apologizing? You're really weird, Ryuuji." (Coalguys sub) What really struck me the most, however, was the scene where Taiga shows Ryuuji how she looks in her swimsuit: she was smiling! And not just any regular smile, but one of those, slight,, coy, corner of the lips, looking-away-cause-I'm-feeling-a-little-shy smiles. It could be that Taiga is being self-deprecating and is amused by her situation, but I'd expect she'd be angry instead of self-pitying. And finally, at the end of the episode, shes' all upset about the incident at the pool. But what is she most upset about? Not that she was seen flailing around the pool, but that Ryuuji touched her--despite the fact that her flatness wasn't revealed to the rest of the class.
Ami changes slightly, too, I think. Whereas before she used to pull the I'm-the-innocent-victim-being-bullied-by-Taiga card, now she's starting to bite back when given the opportunity (at the beginning of class, at the pool) and in front of other people, too. At the swimsuit store, she also gets all full of herself in front of Ryuuji. And finally, I think lifting up Taiga and chucking her into the middle of the pool isn't exactly a very Aww-I'm-such-a-clumsy-Ami-chan thing to do.
And just for the hell of it: