Sunao is attempting suicide by drowning himself in episode 7.Think about it. His life already sucks, what with being abandoned in the lab and all. Now hes going around pretending that everythings all hunky-dory, trying to get close to Sora and the rest, while Aizawa and Kai are rushing him. If that isnt stressful enough as it is, despite anything he may have claimed, he was maybe falling for Sora around this time. So when he goes to the pool, he tries to kill himself. Typically people wouldnt try to teach themselves how to swim by diving straight into the deep end. Then he doesnt even try to swim, and its Ran who shouts for help. Anything said about the incident afterwards is an attempt to justify it to himself or to get closer to Sora.
WMG / Sukisho