The reading for 西 here is an exception. Here's a mnemonic to help you remember it:
Watermelons taste best after you swing (すい) them around. Whether you tie a rope to them or swing them on an actual swing, it mixes up the yummy watermelon insides and they end up tasting the best this way. So always remember to swing your watermelon before you crack it open.
When it comes to summer, watermelon's the first thing that comes to mind.