in the water, swimming in the water

如果要表达水里有某物品，应该用in. 如果要表达把某物品放进水里，则用 into.

比如：

There are some stones in the wate. 水里有一些石头。

Please put these stones into the wate. 请把这些石头放进水里。

因为游泳本身就是在水里的，就不用加“在水里了”： She is swimming. 如果说 She is swimming in the wate, 就有点怪了。