Takao's attempt at drowning himself gives Midorima some time to recover and get back to a more neutral state. It's good, because he's not prepared to think about Takao's ass self-lubricating yet. Midorima's had a similar talk with Kise, but he wasn't as close to Kise back then. Takao is, despite all verbal denials that Midorima may make, Midorima's closest friend. This world tends to make that line very, very confusing.

When he doesn't come up after a certain amount of time, Midorima has a very simple reaction: Three-point shoot the bath sponge at the back of his partner's head. He has no doubt that will get him to resurface. If it doesn't, he'll try a bar of soap. Takao knows he has no problem escalating and making the same shot over and over until he gets what he wants.