They wait until everyone else is heading to the showers or over to the volleyball net on the grass before going into the water. Yamaguchi did go in before, but Tsukki says he waited because he wanted to be able to swim without the risk of being drowned by "any of the idiots or the godda-[choking noises] [huff]—senpai". (Well. Yamaguchi knows that that's true, but he also knows that Tsukki burns after spending like ten seconds in the sun with two layers of sunscreen on, and sun reflecting off the sea makes it worse. It's not pretty to deal with.)
The sun's still softly warm, as is the sand under his feet, and just ahead of him Tsukki's shadow stretches back four, five metres along the sand. He shed the old T-shirt he's been wearing all day just moments ago, and something settles in Yamaguchi's stomach and spreads up into his smile. It's a rarity smaller than the number of fingers he has on one hand, and yes it's selfish but it's also perfect; because this sight—Tsukki's back, bare (beautiful)—is his, all his and only his.
For once, he doesn't run to catch up.
Tsukki is already thigh-deep in the sea when Yamaguchi steps tentatively in. He yelps at the cold, and Tsukki looks over his shoulder. But he doesn't comment, just raises an eyebrow before wading deeper and sinking down to start swimming.
It's not so bad after a minute, and Yamaguchi wades in with more conviction now that the water is bearably cool. He jumps reflexively when a breaking wave swells in towards the shore and grins. It's like being six or seven years old again—being at the beach is always like being seven years old, and it's something he appreciates a lot nowadays. He's navel-deep now, and with a sigh he sinks himself down and then brings his legs up so he's floating on the surface.
He closes his eyes and lets his head sink further into the water. It rises over his ears in a sighing gush, laps gently across his stomach, and supports him in a cool embrace. His stomach and face are warm under the sun, and he slowly feels his body unwind and his muscles relax.
There's a distant splashing near him—though, everything sounds distant with his ears underwater—and that's all the warning he has before there's a hand on his shoulder, fingers long and gently squeezing. Yamaguchi smiles. And then the hand changes its grip, and there's a dip in the water, waves rippling and receding over his chest.
Something presses against the crook of his neck, and there's a head and softly floating hair brushing his. His stomach flutters, and he inhales sharply through his nose—
And then Tsukki's head whacks against his and knocks him forwards. He splashes in the water for several seconds, trying to find his footing through the twinge rattling through his head, and when he finds his footing at last he turns around, wincing through the pain—only to find Tsukki spitting and grimacing and hacking into the water.
"Ugh—blegh—oh god, that was foul—"
His hair is plastered flat to his head and his hands are alternately wiping his mouth and clutching at the base of his throat. Tsukki really looks ridiculous like that—and his prescription goggles aren't helping matters in the slightest—and Yamaguchi sniggers despite himself because really, the intention was very cute but seawater is disgusting to taste, far less swallow, and Yamaguchi's prepared to bet that Tsukki's accidentally done both. What a dork.
"No one hears about this! No one—stop—blegh—stop laughing, Yamaguchi!"