From my point of view it's not because Yozora wanted to ruin Sena's fun time she decided to come. In fact Yozora already noticed the pool will be filled by riajuu she hates during summer. The reason is because what Yozora and Kodaka said at the club room: they go as an exercise/simulation if in the future they get 'real' friends and those friend ask them to go to pool together.
It's already clear Yozora hates riajuu, and she even told Kodaka she couldn't really interact with normal people but she's fine interacting with others who socially-inept or have few friend. That by itself should be understandable if she couldn't stand on the place with too many people, but she still can endure public place like book store, public transportation, etc anyway. What Yozora and Rika couldn't take was the over-crowd on the bus and the pool, that's too much for them.
No one know (or even see it coming) that the pool gave whopping 50% off during summer, just the time when the pool is on its highest demand
(some company would charge more if they were on that kind of condition XD). If the pool weren't that full but Yozora and Rika still decide to leave, I think Kodaka co won't leave immediately since they're already there and Kobato was looking forward to swim, so they could still have little bit fun.
And FYI, I like Sena more than Yozora
