Do want to water at sundown and be reminded then, or earlier? This is important, because it’s either a sun trigger at sundown, or a sun trigger with an offset, or at another fixed time using a time trigger. Those are your options.
The rain bit I would extract as a separate binary sensor from whichever weather integration you have. Probably a template sensor and a weather service call. Then use this binary sensor as a condition.
There are several options for notifications.
Note that it’s generally good to show what you’ve read or tried around here in order to get help.