入水自殺・入水は suicide by drowning といいます。
or
drowned him/her self
例文:
入水自殺のニュースをみました。
I saw on the news that someone committed suicide by drowning.
I saw on the news that someone drowned themselves.
過労のストレスで入水をしたそうです。
It seems like they drowned themselves due to the stress of overwork.
It seems like the committed suicide by drowning themselves due to the stress of overwork.