The readings are the kun'yomi vocab readings. You know 塩's from the kanji and you know 水's from the vocab word 水. Combine them together and you have しおみず.
I thought I had a good nose, but I couldn't tell which one was salt water.
I think puffer fish dipped in salt water and dried in the sun would be pretty tasty.
The temperature itself isn't that high, but the sun's rays are really strong, so why don't you take some salt water to prevent heatstroke?