The war is coming.
Percy sees it as clearly as he saw Luke betray everyone and everything he knows. Tension simmers — Kronos' army is growing daily, and there's nothing he can do stop it. He despises it. Can't he have a normal summer for once? There's always someone who needs defeating, someone who needs saving, someone who's lost something...
Of course, the impending threat of conflict looms over him more than anyone else. He is the most powerful demigod alive, after all, and therefore the weight of this lies on his shoulders, which is exactly why he's savouring the unstable peace.
"D'you wanna go for a swim? We have enough time before dinner," he proposes with an eager look as if he isn't constantly fearing for the lives of his friends and camp mates. Percy is fully aware that he won't be able to enjoy luxuries like relaxing when the dreaded time comes.