jì is the everyday reading: 经济 (economy), 救济 (relief, aid), 无济于事 (to no avail). It carries two old senses - to cross a river and to help people across hardship. The jǐ reading is separate: it names the city 济南 and appears in 人才济济 (a galaxy of talent).
The three-drop water radical, a left-side form of 水, places 济 among crossing and flowing characters - the original sense was fording a river. The same radical anchors 河, 海 and 洗, marking anything liquid or watery.
Right side supplies the sound: 齐 (qí) drifted to jì, a common initial-and-tone shift. It also lends a faint flavor of 'all together, in step', which fits 济's sense of crossing as a group or aiding many at once.
