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No, not necessarily, but it can cause a minor issue. iPhones use a Lithium-Ion battery for its power, using while it's charging won't necessarily ruin the battery, but if you're using the phone while the battery is in the charging process, it can cause the phone to think it has more power than it actually has.
When your cell phone falls in the water, you don't necessarily need to start shopping for a replacement right away. If you quickly dry out the phone and its battery, the device may survive the experience. If you do not dry the battery properly, however, water can damage and corrode its sensitive components. Many cell phones have batteries that feature a moisture indicator, which is a sticker that alerts you to water damage.