I never could get a feel for opening my eyes while under water.

The eye... it opened!

I like how there's a huge glass container for water just sitting somewhere in the house to swim in.

Since I'm getting used to some advanced kanji, 目 can mean eyes as Utsuho said, or just "eye" as Satori-san thought it meant. But in the last panel, Koichi-chan's eye is peeking.

Koishi trolling as usual.