Rei's bellyflop into the Samezuka pool at his first swim practice.
Not to mention that he wasn't intending to swim in the first place (being unable to swim and all), and is forced into a tiny Speedo in order to do said bellyflop.
Speaking of Rei, there's also his ridiculous fashion sense during the shopping trip, including a butterfly themed swimsuit (complete with wings). Busou Renkin might suddenly come to mind.
Even funnier is that when he's showing the butterfly suit off, Makoto shouts, "Where did you find that?!" offscreen.
Haruka's attempt to get someone to join the swimming club so he could swim through the whole year. It fails immediately.
Nagisa's attempts to use Amakata to get more members into the swim club.
The first time, he suggests they get members to join by promising them that they'll see her in a swimsuit. Cue a pissed Amakata standing behind him, sporting a black Battle Aura and letting out a long 'Excuse me?!?'.
The second time, he says that instead of showing off Amakata in a swimsuit to the new members, the members will show off their swimsuits to her. Cue a shot of her standing behind them, sporting a creepy smile that would do many a Stringy-Haired Ghost Girl proud.
Makoto and Nagisa: WE'RE SO SORRY!
This exchange from episode 3. Haruka's so offended and protective of the water!
Nagisa: I don't think Rei-chan likes water.
Haruka: We don't need anyone like that. Don't let him in the water.
The shopping trip in episode 4. The things they try on have to be seen to be believed. Especially Rei's.
Rei: I'm accounting for each color having a different psychological impact
Makoto: So, you don't care what it looks like?
Rei sounds so proud of himself for finding the suit too.
The fact that no one can tell the difference between the suits Haru tries on and the suits he already has. He insists he prefers the fit of what he tries on, though. Something about seeing him glance intently between two (seemingly?) identical pairs of trunks is worth a grin as well.
Throughout this fashion show, Kou's fangirling gets progressively less enthusiastic until she's fallen asleep by the end of it.
As it turns out, Haru has a selection of swimming trunks to choose from. The best part? They're all exactly the same, sans a slightly different pattern on each and, very important, they all fit different.
The animators at Kyo Ani must have had a lot of fun with episode 4. Special mention goes to thatbutt clench. Goodness.
Rei's swimming failures in episode 4, especially when he tries to do the backstroke. When he did, less and less arm stuck out of the water, until only a small ripple on the surface remained.
Relating to above, when other characters discuss why Rei can't swim Haruka suggests that water doesn't like him. Cue Nagisa actually pitying the poor guy.
One that might not be so obvious: in episode five, Rei attempts to adjust his glasses..... then realizes that he's not wearing his glasses.
In episode 6, the Iwatobi boys play a truth-or-dare type game, and Haru is told to tell about an experience with love. He tells a story about the first time he saw a waterfall.
So much of the drama cd. Special mention goes to Rin having to wear a maid outfit.
Gou having extensive notes on the rival competitors muscles. Gou fangirling over muscles in general.
Bonus points for the boys not even reacting to her. They've gotten so used to her now.
The scene from episode 9 when Haru is playing the yo-yo fishing game and has already won at least five yo-yo's, but he keeps playing is funny for some reason. It probably has to do with his completely deadpan expression as he plays.
Ninja/Detective Rei from episode 9. Also his dramatic 'Sky High!' pose from earlier in the episode.
The preview for episode 10 has Rei and and Rin fiercely competing to see which one can pronounce "all out" better.
Making it even more hilarious, neither can pronounce it recognizably.
"Are you Rin-chan-san?"
The entirety of the FrFr! Special extra from the DVD/Blu-Ray version.
Rin's Engrish and Neat Freak tendencies in the second Fr Fr! short. Special mention goes to his reaction to Nitori's preserved umbilical cord and porn mag.