The primary meaning of drown is to die by being underwater too long. "Drowned to death" is unnatural. Oxford Learner's Dictionaries https://www.oxfordlearnersdictionaries.com/definition/english/drown drown verb ​[intransitive, transitive] to die because you have been underwater too long and you cannot breathe; to kill somebody in this way Two children drowned after falling into the river. He had attempted to rescue the drowning man. drown somebody/something/yourself She tried to drown herself. He was drowned at sea. They had drowned the unwanted kittens.

Drown - death from suffocation in a fluid (liquid or gas). "They were drowned in a sea of carbon dioxide. No. It would be redundant.