Three lines are all it takes to put you in the middle of a sweltering day. The poem names a situation anyone can recognize: it's hot, it's summer, and there's nothing cold to drink. The speaker doesn't say "I'm thirsty" or "I'm miserable." We work that out ourselves, and that's part of what makes the poem work.
The structure helps. The first two lines set up the heat, almost lazily, and then the last line lands with a flat, blunt "no bottled water." The dash before it acts like a small pause, a moment where the speaker realizes what's missing. The poem is so short that it feels like a sigh, or a muttered complaint.
The mood is dry, irritated, and a little wry. Nothing dramatic happens, and the poem seems to know that. It captures a small, ordinary frustration, and by keeping the words so sparse, it makes you feel the same scarcity the speaker does.