What is the difference between 降水 and 增水?
They refer to different things. 降水 means rainfall; but 增水 means 风暴增水(风暴潮) here, from which coastal cities
might suffer. e.g. from this news 台风“卡努”：海口今夜至明晨将出现风暴增水,

人民网讯 受台风“卡努”外围环流影响，近三小时，海口全市普降小到中雨，最大累积雨量出现在新埠岛13.8毫米。10月15日潮位：最高潮在上午11点53分，是2.27米，最低潮在凌晨零点51分，是0.82米。预计今天夜间到明天凌晨，海口一带沿岸将出现80～150cm的风暴增水。

风暴增水/风暴潮

风暴潮或称暴潮（英语：storm surge）是由热带气旋、温带气旋、冷锋的强风作用和气压骤变等强烈的天气系统引起的海面异常升降现象，又称“风暴增水”、“风暴海啸”、“气象海啸”或“风潮”。风暴潮会使受到影响的海区的潮位大大地超过正常。如果风暴潮恰好与影响海区涨潮相重叠，就会使水位暴涨，海水涌进内陆，造成巨大破坏。
A storm surge or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (such as tropical cyclones and strong extratropical cyclones), the severity of which is affected by the shallowness and orientation of the water body relative to storm path, as well as the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges. It is a measure of the rise of water beyond what would be expected by the normal movement related to tides.
The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, and a low-pressure-induced dome of water drawn up under and trailing the storm's center.