I know that the 해라체 is used in diaries, books, newspapers and so forth, but is it also used for casual personal notes to myself? Like "water the flowers" or statements like "password is 123".
It’s okay to use 해라체 in personal notes but in general it isn’t preferred. They’d rather write sentences without 상대 높임법 applied. A way to do that is to use 명사형 종결어미 like
~음 which makes the sentence sound very neutral.
Afaik the most common form in 'TODO' sentences is
~하기. I also use it myself in my personal notes. For example,
화분에 물 주기: Water the flowers.
빨래 널기: Do the laundry.
일찍 일어나기: Wake up early.
And I’d use
~음 for any sentence that simply states facts or observations.
날씨 맑음: It’s sunny.
날씨가 흐림: It’s cloudy.
암호는 123임: Password is 123.
열쇠는 상자 안에 있음: The key is in the box.
적에게 기습 공격 받음: The enemy made a surprise attack.